QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Jens,

I'm experimenting here with trying to generate large I/O through libata,
and not having much luck.

The limit seems to be the number of hardware PRD (SG) entries permitted
by the driver (libata:ata_piix), which is 128 by default.

The problem is, the block layer *never* sends an SG entry larger than 8192 bytes,
and even that size is exceptionally rare.  Nearly all I/O segments are 4096 bytes,
so I never see a single I/O larger than 512KB (128 * 4096).

If I patch various parts of block and SCSI, this limit doesn't budge,
but when I change the hardware PRD limit in libata, it scales by exactly
whatever I set the new value to.  This tells me that adjacent I/O segments
are not being combined.

I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should
result in adjacent single pages being combined into larger physical segments?

This is x86-32 with latest 2.6.24-rc*.
I'll re-test on older kernels next.

???

QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

(resending with corrected email address for Jens)

Jens,

I'm experimenting here with trying to generate large I/O through libata,
and not having much luck.

The limit seems to be the number of hardware PRD (SG) entries permitted
by the driver (libata:ata_piix), which is 128 by default.

The problem is, the block layer *never* sends an SG entry larger than 8192 bytes,
and even that size is exceptionally rare.  Nearly all I/O segments are 4096 bytes,
so I never see a single I/O larger than 512KB (128 * 4096).

If I patch various parts of block and SCSI, this limit doesn't budge,
but when I change the hardware PRD limit in libata, it scales by exactly
whatever I set the new value to.  This tells me that adjacent I/O segments
are not being combined.

I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should
result in adjacent single pages being combined into larger physical segments?

This is x86-32 with latest 2.6.24-rc*.
I'll re-test on older kernels next.

???

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Matthew Wilcox]

On Thu, Dec 13, 2007 at 01:37:59PM -0500, Mark Lord wrote:
> The problem is, the block layer *never* sends an SG entry larger than 8192 
> bytes,
> and even that size is exceptionally rare.  Nearly all I/O segments are 4096 
> bytes,
> so I never see a single I/O larger than 512KB (128 * 4096).
> 
> If I patch various parts of block and SCSI, this limit doesn't budge,
> but when I change the hardware PRD limit in libata, it scales by exactly
> whatever I set the new value to.  This tells me that adjacent I/O segments
> are not being combined.
> 
> I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should
> result in adjacent single pages being combined into larger physical 
> segments?

I was recently debugging a driver and noticed that consecutive pages in
an sg list are in the reverse order.  ie first you get page 918, then
917, 916, 915, 914, etc.  I vaguely remember James having patches to
correct this, but maybe they weren't merged?

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[James Bottomley]

On Thu, 2007-12-13 at 11:42 -0700, Matthew Wilcox wrote:
> On Thu, Dec 13, 2007 at 01:37:59PM -0500, Mark Lord wrote:
> > The problem is, the block layer *never* sends an SG entry larger than 8192 
> > bytes,
> > and even that size is exceptionally rare.  Nearly all I/O segments are 4096 
> > bytes,
> > so I never see a single I/O larger than 512KB (128 * 4096).
> > 
> > If I patch various parts of block and SCSI, this limit doesn't budge,
> > but when I change the hardware PRD limit in libata, it scales by exactly
> > whatever I set the new value to.  This tells me that adjacent I/O segments
> > are not being combined.
> > 
> > I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should
> > result in adjacent single pages being combined into larger physical 
> > segments?
> 
> I was recently debugging a driver and noticed that consecutive pages in
> an sg list are in the reverse order.  ie first you get page 918, then
> 917, 916, 915, 914, etc.  I vaguely remember James having patches to
> correct this, but maybe they weren't merged?

Yes, they were ... it was actually Bill Irwin's patch.  The old problem
was that we fault allocations in reverse order (because we were taking
from the end of the zone list).  I can't remember when his patches went
in, but it was several years ago.  After they did, I was getting a 33%
chance of physical merging (as opposed to zero before).  Probably
someone redid the vm or the zones without understanding this and we've
gone back to the original position.

James

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mark Lord wrote:
> (resending with corrected email address for Jens)
> 
> Jens,
> 
> I'm experimenting here with trying to generate large I/O through libata,
> and not having much luck.
> 
> The limit seems to be the number of hardware PRD (SG) entries permitted
> by the driver (libata:ata_piix), which is 128 by default.
> 
> The problem is, the block layer *never* sends an SG entry larger than 
> 8192 bytes,
> and even that size is exceptionally rare.  Nearly all I/O segments are 
> 4096 bytes,
> so I never see a single I/O larger than 512KB (128 * 4096).
> 
> If I patch various parts of block and SCSI, this limit doesn't budge,
> but when I change the hardware PRD limit in libata, it scales by exactly
> whatever I set the new value to.  This tells me that adjacent I/O segments
> are not being combined.
> 
> I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should
> result in adjacent single pages being combined into larger physical 
> segments?
> 
> This is x86-32 with latest 2.6.24-rc*.
> I'll re-test on older kernels next.
...

Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB for libata,
but 2.6.24 uses only 4KB segments and a *few* 8KB segments.

???

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Matthew Wilcox]

On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB for 
> libata,
> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.

Just a suspicion ... could this be slab vs slub?  ie check your configs
are the same / similar between the two kernels.

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Matthew Wilcox wrote:
> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>> Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB for 
>> libata,
>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> 
> Just a suspicion ... could this be slab vs slub?  ie check your configs
> are the same / similar between the two kernels.
..

Mmmm.. a good thought, that one.
But I just rechecked, and both have CONFIG_SLAB=y

My guess is that something got changed around when Jens
reworked the block layer for 2.6.24.
I'm going to dig around in there now.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Mark Lord wrote:
> Matthew Wilcox wrote:
> >On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB for 
> >>libata,
> >>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >
> >Just a suspicion ... could this be slab vs slub?  ie check your configs
> >are the same / similar between the two kernels.
> ..
> 
> Mmmm.. a good thought, that one.
> But I just rechecked, and both have CONFIG_SLAB=y
> 
> My guess is that something got changed around when Jens
> reworked the block layer for 2.6.24.
> I'm going to dig around in there now.

I didn't rework the block layer for 2.6.24 :-). The core block layer
changes since 2.6.23 are:

- Support for empty barriers. Not a likely candidate.
- Shared tag queue fixes. Totally unlikely.
- sg chaining support. Not likely.
- The bio changes from Neil. Of the bunch, the most likely suspects in
  this area, since it changes some of the code involved with merges and
  blk_rq_map_sg().
- Lots of simple stuff, again very unlikely.

Anyway, it sounds odd for this to be a block layer problem if you do see
occasional segments being merged. So it sounds more like the input data
having changed.

Why not just bisect it?

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
>> Matthew Wilcox wrote:
>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB for 
>>>> libata,
>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>> Just a suspicion ... could this be slab vs slub?  ie check your configs
>>> are the same / similar between the two kernels.
>> ..
>>
>> Mmmm.. a good thought, that one.
>> But I just rechecked, and both have CONFIG_SLAB=y
>>
>> My guess is that something got changed around when Jens
>> reworked the block layer for 2.6.24.
>> I'm going to dig around in there now.
> 
> I didn't rework the block layer for 2.6.24 :-). The core block layer
> changes since 2.6.23 are:
> 
> - Support for empty barriers. Not a likely candidate.
> - Shared tag queue fixes. Totally unlikely.
> - sg chaining support. Not likely.
> - The bio changes from Neil. Of the bunch, the most likely suspects in
>   this area, since it changes some of the code involved with merges and
>   blk_rq_map_sg().
> - Lots of simple stuff, again very unlikely.
> 
> Anyway, it sounds odd for this to be a block layer problem if you do see
> occasional segments being merged. So it sounds more like the input data
> having changed.
> 
> Why not just bisect it?
..

Because the early 2.6.24 series failed to boot on this machine
due to bugs in the block layer -- so the code that caused this regression
is probably in the stuff from before the kernels became usable here.

Cheers

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mark Lord wrote:
> Jens Axboe wrote:
>> On Thu, Dec 13 2007, Mark Lord wrote:
>>> Matthew Wilcox wrote:
>>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 
>>>>> 64KB for libata,
>>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>>> Just a suspicion ... could this be slab vs slub?  ie check your configs
>>>> are the same / similar between the two kernels.
>>> ..
>>>
>>> Mmmm.. a good thought, that one.
>>> But I just rechecked, and both have CONFIG_SLAB=y
>>>
>>> My guess is that something got changed around when Jens
>>> reworked the block layer for 2.6.24.
>>> I'm going to dig around in there now.
>>
>> I didn't rework the block layer for 2.6.24 :-). The core block layer
>> changes since 2.6.23 are:
>>
>> - Support for empty barriers. Not a likely candidate.
>> - Shared tag queue fixes. Totally unlikely.
>> - sg chaining support. Not likely.
>> - The bio changes from Neil. Of the bunch, the most likely suspects in
>>   this area, since it changes some of the code involved with merges and
>>   blk_rq_map_sg().
>> - Lots of simple stuff, again very unlikely.
>>
>> Anyway, it sounds odd for this to be a block layer problem if you do see
>> occasional segments being merged. So it sounds more like the input data
>> having changed.
>>
>> Why not just bisect it?
> ..
> 
> Because the early 2.6.24 series failed to boot on this machine
> due to bugs in the block layer -- so the code that caused this regression
> is probably in the stuff from before the kernels became usable here.
..

That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
the first couple of -rc* ones failed here because of incompatibilities
between the block/bio changes and libata.

That's better, I think! 

Cheers

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Mark Lord wrote:
> Mark Lord wrote:
> >Jens Axboe wrote:
> >>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>Matthew Wilcox wrote:
> >>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> >>>>>64KB for libata,
> >>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>>Just a suspicion ... could this be slab vs slub?  ie check your configs
> >>>>are the same / similar between the two kernels.
> >>>..
> >>>
> >>>Mmmm.. a good thought, that one.
> >>>But I just rechecked, and both have CONFIG_SLAB=y
> >>>
> >>>My guess is that something got changed around when Jens
> >>>reworked the block layer for 2.6.24.
> >>>I'm going to dig around in there now.
> >>
> >>I didn't rework the block layer for 2.6.24 :-). The core block layer
> >>changes since 2.6.23 are:
> >>
> >>- Support for empty barriers. Not a likely candidate.
> >>- Shared tag queue fixes. Totally unlikely.
> >>- sg chaining support. Not likely.
> >>- The bio changes from Neil. Of the bunch, the most likely suspects in
> >>  this area, since it changes some of the code involved with merges and
> >>  blk_rq_map_sg().
> >>- Lots of simple stuff, again very unlikely.
> >>
> >>Anyway, it sounds odd for this to be a block layer problem if you do see
> >>occasional segments being merged. So it sounds more like the input data
> >>having changed.
> >>
> >>Why not just bisect it?
> >..
> >
> >Because the early 2.6.24 series failed to boot on this machine
> >due to bugs in the block layer -- so the code that caused this regression
> >is probably in the stuff from before the kernels became usable here.
> ..
> 
> That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
> the first couple of -rc* ones failed here because of incompatibilities
> between the block/bio changes and libata.
> 
> That's better, I think! 

No worries, I didn't pick it up as harsh just as an odd conclusion :-)

If I were you, I'd just start from the first -rc that booted for you. If
THAT has the bug, then we'll think of something else. If you don't get
anywhere, I can run some tests tomorrow and see if I can reproduce it
here.

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
>> Mark Lord wrote:
>>> Jens Axboe wrote:
>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>> Matthew Wilcox wrote:
>>>>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 
>>>>>>> 64KB for libata,
>>>>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>>>>> Just a suspicion ... could this be slab vs slub?  ie check your configs
>>>>>> are the same / similar between the two kernels.
>>>>> ..
>>>>>
>>>>> Mmmm.. a good thought, that one.
>>>>> But I just rechecked, and both have CONFIG_SLAB=y
>>>>>
>>>>> My guess is that something got changed around when Jens
>>>>> reworked the block layer for 2.6.24.
>>>>> I'm going to dig around in there now.
>>>> I didn't rework the block layer for 2.6.24 :-). The core block layer
>>>> changes since 2.6.23 are:
>>>>
>>>> - Support for empty barriers. Not a likely candidate.
>>>> - Shared tag queue fixes. Totally unlikely.
>>>> - sg chaining support. Not likely.
>>>> - The bio changes from Neil. Of the bunch, the most likely suspects in
>>>>  this area, since it changes some of the code involved with merges and
>>>>  blk_rq_map_sg().
>>>> - Lots of simple stuff, again very unlikely.
>>>>
>>>> Anyway, it sounds odd for this to be a block layer problem if you do see
>>>> occasional segments being merged. So it sounds more like the input data
>>>> having changed.
>>>>
>>>> Why not just bisect it?
>>> ..
>>>
>>> Because the early 2.6.24 series failed to boot on this machine
>>> due to bugs in the block layer -- so the code that caused this regression
>>> is probably in the stuff from before the kernels became usable here.
>> ..
>>
>> That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
>> the first couple of -rc* ones failed here because of incompatibilities
>> between the block/bio changes and libata.
>>
>> That's better, I think! 
> 
> No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> 
> If I were you, I'd just start from the first -rc that booted for you. If
> THAT has the bug, then we'll think of something else. If you don't get
> anywhere, I can run some tests tomorrow and see if I can reproduce it
> here.
..

I believe that *anyone* can reproduce it, since it's broken long before
the requests ever get to SCSI or libata.  Which also means that *anyone*
who wants to can bisect it, as well.

I don't do "bisects".

But I will dig a bit more and see if I can find the culprit.

Cheers

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Mark Lord wrote:
> >>Mark Lord wrote:
> >>>Jens Axboe wrote:
> >>>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>>Matthew Wilcox wrote:
> >>>>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> >>>>>>>64KB for libata,
> >>>>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>>>>Just a suspicion ... could this be slab vs slub?  ie check your 
> >>>>>>configs
> >>>>>>are the same / similar between the two kernels.
> >>>>>..
> >>>>>
> >>>>>Mmmm.. a good thought, that one.
> >>>>>But I just rechecked, and both have CONFIG_SLAB=y
> >>>>>
> >>>>>My guess is that something got changed around when Jens
> >>>>>reworked the block layer for 2.6.24.
> >>>>>I'm going to dig around in there now.
> >>>>I didn't rework the block layer for 2.6.24 :-). The core block layer
> >>>>changes since 2.6.23 are:
> >>>>
> >>>>- Support for empty barriers. Not a likely candidate.
> >>>>- Shared tag queue fixes. Totally unlikely.
> >>>>- sg chaining support. Not likely.
> >>>>- The bio changes from Neil. Of the bunch, the most likely suspects in
> >>>> this area, since it changes some of the code involved with merges and
> >>>> blk_rq_map_sg().
> >>>>- Lots of simple stuff, again very unlikely.
> >>>>
> >>>>Anyway, it sounds odd for this to be a block layer problem if you do see
> >>>>occasional segments being merged. So it sounds more like the input data
> >>>>having changed.
> >>>>
> >>>>Why not just bisect it?
> >>>..
> >>>
> >>>Because the early 2.6.24 series failed to boot on this machine
> >>>due to bugs in the block layer -- so the code that caused this regression
> >>>is probably in the stuff from before the kernels became usable here.
> >>..
> >>
> >>That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
> >>the first couple of -rc* ones failed here because of incompatibilities
> >>between the block/bio changes and libata.
> >>
> >>That's better, I think! 
> >
> >No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> >
> >If I were you, I'd just start from the first -rc that booted for you. If
> >THAT has the bug, then we'll think of something else. If you don't get
> >anywhere, I can run some tests tomorrow and see if I can reproduce it
> >here.
> ..
> 
> I believe that *anyone* can reproduce it, since it's broken long before
> the requests ever get to SCSI or libata.  Which also means that *anyone*
> who wants to can bisect it, as well.
> 
> I don't do "bisects".

It was just a suggestion on how to narrow it down, do as you see fit.

> But I will dig a bit more and see if I can find the culprit.

Sure, I'll dig around as well.

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
>> Jens Axboe wrote:
>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>> Mark Lord wrote:
>>>>> Jens Axboe wrote:
>>>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>>>> Matthew Wilcox wrote:
>>>>>>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>>>>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 
>>>>>>>>> 64KB for libata,
>>>>>>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>>>>>>> Just a suspicion ... could this be slab vs slub?  ie check your 
>>>>>>>> configs
>>>>>>>> are the same / similar between the two kernels.
>>>>>>> ..
>>>>>>>
>>>>>>> Mmmm.. a good thought, that one.
>>>>>>> But I just rechecked, and both have CONFIG_SLAB=y
>>>>>>>
>>>>>>> My guess is that something got changed around when Jens
>>>>>>> reworked the block layer for 2.6.24.
>>>>>>> I'm going to dig around in there now.
>>>>>> I didn't rework the block layer for 2.6.24 :-). The core block layer
>>>>>> changes since 2.6.23 are:
>>>>>>
>>>>>> - Support for empty barriers. Not a likely candidate.
>>>>>> - Shared tag queue fixes. Totally unlikely.
>>>>>> - sg chaining support. Not likely.
>>>>>> - The bio changes from Neil. Of the bunch, the most likely suspects in
>>>>>> this area, since it changes some of the code involved with merges and
>>>>>> blk_rq_map_sg().
>>>>>> - Lots of simple stuff, again very unlikely.
>>>>>>
>>>>>> Anyway, it sounds odd for this to be a block layer problem if you do see
>>>>>> occasional segments being merged. So it sounds more like the input data
>>>>>> having changed.
>>>>>>
>>>>>> Why not just bisect it?
>>>>> ..
>>>>>
>>>>> Because the early 2.6.24 series failed to boot on this machine
>>>>> due to bugs in the block layer -- so the code that caused this regression
>>>>> is probably in the stuff from before the kernels became usable here.
>>>> ..
>>>>
>>>> That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
>>>> the first couple of -rc* ones failed here because of incompatibilities
>>>> between the block/bio changes and libata.
>>>>
>>>> That's better, I think! 
>>> No worries, I didn't pick it up as harsh just as an odd conclusion :-)
>>>
>>> If I were you, I'd just start from the first -rc that booted for you. If
>>> THAT has the bug, then we'll think of something else. If you don't get
>>> anywhere, I can run some tests tomorrow and see if I can reproduce it
>>> here.
>> ..
>>
>> I believe that *anyone* can reproduce it, since it's broken long before
>> the requests ever get to SCSI or libata.  Which also means that *anyone*
>> who wants to can bisect it, as well.
>>
>> I don't do "bisects".
> 
> It was just a suggestion on how to narrow it down, do as you see fit.
> 
>> But I will dig a bit more and see if I can find the culprit.
> 
> Sure, I'll dig around as well.
..

I wonder if it's 9dfa52831e96194b8649613e3131baa2c109f7dc:
     "Merge blk_recount_segments into blk_recalc_rq_segments" ?

That particular commit does some rather innocent code-shuffling,
but also introduces a couple of new "if (nr_hw_segs == 1" conditions
that were not there before.

Okay git experts:  how do I pull out a kernel at the point of this exact commit ?

Thanks!

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Mark Lord wrote:
> >>Jens Axboe wrote:
> >>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>Mark Lord wrote:
> >>>>>Jens Axboe wrote:
> >>>>>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>>>>Matthew Wilcox wrote:
> >>>>>>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>>>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> >>>>>>>>>64KB for libata,
> >>>>>>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>>>>>>Just a suspicion ... could this be slab vs slub?  ie check your 
> >>>>>>>>configs
> >>>>>>>>are the same / similar between the two kernels.
> >>>>>>>..
> >>>>>>>
> >>>>>>>Mmmm.. a good thought, that one.
> >>>>>>>But I just rechecked, and both have CONFIG_SLAB=y
> >>>>>>>
> >>>>>>>My guess is that something got changed around when Jens
> >>>>>>>reworked the block layer for 2.6.24.
> >>>>>>>I'm going to dig around in there now.
> >>>>>>I didn't rework the block layer for 2.6.24 :-). The core block layer
> >>>>>>changes since 2.6.23 are:
> >>>>>>
> >>>>>>- Support for empty barriers. Not a likely candidate.
> >>>>>>- Shared tag queue fixes. Totally unlikely.
> >>>>>>- sg chaining support. Not likely.
> >>>>>>- The bio changes from Neil. Of the bunch, the most likely suspects in
> >>>>>>this area, since it changes some of the code involved with merges and
> >>>>>>blk_rq_map_sg().
> >>>>>>- Lots of simple stuff, again very unlikely.
> >>>>>>
> >>>>>>Anyway, it sounds odd for this to be a block layer problem if you do 
> >>>>>>see
> >>>>>>occasional segments being merged. So it sounds more like the input 
> >>>>>>data
> >>>>>>having changed.
> >>>>>>
> >>>>>>Why not just bisect it?
> >>>>>..
> >>>>>
> >>>>>Because the early 2.6.24 series failed to boot on this machine
> >>>>>due to bugs in the block layer -- so the code that caused this 
> >>>>>regression
> >>>>>is probably in the stuff from before the kernels became usable here.
> >>>>..
> >>>>
> >>>>That sounds more harsh than intended --> the earlier 2.6.24 kernels (up 
> >>>>to
> >>>>the first couple of -rc* ones failed here because of incompatibilities
> >>>>between the block/bio changes and libata.
> >>>>
> >>>>That's better, I think! 
> >>>No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> >>>
> >>>If I were you, I'd just start from the first -rc that booted for you. If
> >>>THAT has the bug, then we'll think of something else. If you don't get
> >>>anywhere, I can run some tests tomorrow and see if I can reproduce it
> >>>here.
> >>..
> >>
> >>I believe that *anyone* can reproduce it, since it's broken long before
> >>the requests ever get to SCSI or libata.  Which also means that *anyone*
> >>who wants to can bisect it, as well.
> >>
> >>I don't do "bisects".
> >
> >It was just a suggestion on how to narrow it down, do as you see fit.
> >
> >>But I will dig a bit more and see if I can find the culprit.
> >
> >Sure, I'll dig around as well.
> ..
> 
> I wonder if it's 9dfa52831e96194b8649613e3131baa2c109f7dc:
>     "Merge blk_recount_segments into blk_recalc_rq_segments" ?
> 
> That particular commit does some rather innocent code-shuffling,
> but also introduces a couple of new "if (nr_hw_segs == 1" conditions
> that were not there before.

You can try and revert it of course, but I think you are looking at the
wrong bits. If the segment counts were totally off, you'd never be
anywhere close to reaching the set limit. Your problems seems to be
missed contig segment merges.

> Okay git experts:  how do I pull out a kernel at the point of this exact 
> commit ?

Dummy approach - git log and grep for
9dfa52831e96194b8649613e3131baa2c109f7dc, then see what commit is before
that. Then do a git checkout commit.

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
> > Jens Axboe wrote:
> > >On Thu, Dec 13 2007, Mark Lord wrote:
> > >>Mark Lord wrote:
> > >>>Jens Axboe wrote:
> > >>>>On Thu, Dec 13 2007, Mark Lord wrote:
> > >>>>>Matthew Wilcox wrote:
> > >>>>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> > >>>>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> > >>>>>>>64KB for libata,
> > >>>>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> > >>>>>>Just a suspicion ... could this be slab vs slub?  ie check your 
> > >>>>>>configs
> > >>>>>>are the same / similar between the two kernels.
> > >>>>>..
> > >>>>>
> > >>>>>Mmmm.. a good thought, that one.
> > >>>>>But I just rechecked, and both have CONFIG_SLAB=y
> > >>>>>
> > >>>>>My guess is that something got changed around when Jens
> > >>>>>reworked the block layer for 2.6.24.
> > >>>>>I'm going to dig around in there now.
> > >>>>I didn't rework the block layer for 2.6.24 :-). The core block layer
> > >>>>changes since 2.6.23 are:
> > >>>>
> > >>>>- Support for empty barriers. Not a likely candidate.
> > >>>>- Shared tag queue fixes. Totally unlikely.
> > >>>>- sg chaining support. Not likely.
> > >>>>- The bio changes from Neil. Of the bunch, the most likely suspects in
> > >>>> this area, since it changes some of the code involved with merges and
> > >>>> blk_rq_map_sg().
> > >>>>- Lots of simple stuff, again very unlikely.
> > >>>>
> > >>>>Anyway, it sounds odd for this to be a block layer problem if you do see
> > >>>>occasional segments being merged. So it sounds more like the input data
> > >>>>having changed.
> > >>>>
> > >>>>Why not just bisect it?
> > >>>..
> > >>>
> > >>>Because the early 2.6.24 series failed to boot on this machine
> > >>>due to bugs in the block layer -- so the code that caused this regression
> > >>>is probably in the stuff from before the kernels became usable here.
> > >>..
> > >>
> > >>That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
> > >>the first couple of -rc* ones failed here because of incompatibilities
> > >>between the block/bio changes and libata.
> > >>
> > >>That's better, I think! 
> > >
> > >No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> > >
> > >If I were you, I'd just start from the first -rc that booted for you. If
> > >THAT has the bug, then we'll think of something else. If you don't get
> > >anywhere, I can run some tests tomorrow and see if I can reproduce it
> > >here.
> > ..
> > 
> > I believe that *anyone* can reproduce it, since it's broken long before
> > the requests ever get to SCSI or libata.  Which also means that *anyone*
> > who wants to can bisect it, as well.
> > 
> > I don't do "bisects".
> 
> It was just a suggestion on how to narrow it down, do as you see fit.
> 
> > But I will dig a bit more and see if I can find the culprit.
> 
> Sure, I'll dig around as well.

Just tried something simple. I only see one 12kb segment so far, so not
a lot by any stretch. I also DONT see any missed merges signs, so it
would appear that the pages in the request are simply not contigious
physically.

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e30b1a4..1e34b6f 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1330,6 +1330,8 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 				goto new_segment;
 
 			sg->length += nbytes;
+			if (sg->length > 8192)
+				printk("sg_len=%d\n", sg->length);
 		} else {
 new_segment:
 			if (!sg)
@@ -1349,6 +1351,8 @@ new_segment:
 				sg = sg_next(sg);
 			}
 
+			if (bvprv && (page_address(bvprv->bv_page) + bvprv->bv_len == page_address(bvec->bv_page)))
+				printk("missed merge\n");
 			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
 			nsegs++;
 		}

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Jens Axboe wrote:
> On Thu, Dec 13 2007, Jens Axboe wrote:
>> On Thu, Dec 13 2007, Mark Lord wrote:
>>> Jens Axboe wrote:
>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>> Mark Lord wrote:
>>>>>> Jens Axboe wrote:
>>>>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>>>>> Matthew Wilcox wrote:
>>>>>>>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>>>>>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 
>>>>>>>>>> 64KB for libata,
>>>>>>>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>>>>>>>> Just a suspicion ... could this be slab vs slub?  ie check your 
>>>>>>>>> configs
>>>>>>>>> are the same / similar between the two kernels.
>>>>>>>> ..
>>>>>>>>
>>>>>>>> Mmmm.. a good thought, that one.
>>>>>>>> But I just rechecked, and both have CONFIG_SLAB=y
>>>>>>>>
>>>>>>>> My guess is that something got changed around when Jens
>>>>>>>> reworked the block layer for 2.6.24.
>>>>>>>> I'm going to dig around in there now.
>>>>>>> I didn't rework the block layer for 2.6.24 :-). The core block layer
>>>>>>> changes since 2.6.23 are:
>>>>>>>
>>>>>>> - Support for empty barriers. Not a likely candidate.
>>>>>>> - Shared tag queue fixes. Totally unlikely.
>>>>>>> - sg chaining support. Not likely.
>>>>>>> - The bio changes from Neil. Of the bunch, the most likely suspects in
>>>>>>> this area, since it changes some of the code involved with merges and
>>>>>>> blk_rq_map_sg().
>>>>>>> - Lots of simple stuff, again very unlikely.
>>>>>>>
>>>>>>> Anyway, it sounds odd for this to be a block layer problem if you do see
>>>>>>> occasional segments being merged. So it sounds more like the input data
>>>>>>> having changed.
>>>>>>>
>>>>>>> Why not just bisect it?
>>>>>> ..
>>>>>>
>>>>>> Because the early 2.6.24 series failed to boot on this machine
>>>>>> due to bugs in the block layer -- so the code that caused this regression
>>>>>> is probably in the stuff from before the kernels became usable here.
>>>>> ..
>>>>>
>>>>> That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
>>>>> the first couple of -rc* ones failed here because of incompatibilities
>>>>> between the block/bio changes and libata.
>>>>>
>>>>> That's better, I think! 
>>>> No worries, I didn't pick it up as harsh just as an odd conclusion :-)
>>>>
>>>> If I were you, I'd just start from the first -rc that booted for you. If
>>>> THAT has the bug, then we'll think of something else. If you don't get
>>>> anywhere, I can run some tests tomorrow and see if I can reproduce it
>>>> here.
>>> ..
>>>
>>> I believe that *anyone* can reproduce it, since it's broken long before
>>> the requests ever get to SCSI or libata.  Which also means that *anyone*
>>> who wants to can bisect it, as well.
>>>
>>> I don't do "bisects".
>> It was just a suggestion on how to narrow it down, do as you see fit.
>>
>>> But I will dig a bit more and see if I can find the culprit.
>> Sure, I'll dig around as well.
> 
> Just tried something simple. I only see one 12kb segment so far, so not
> a lot by any stretch. I also DONT see any missed merges signs, so it
> would appear that the pages in the request are simply not contigious
> physically.
> 
> diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> index e30b1a4..1e34b6f 100644
> --- a/block/ll_rw_blk.c
> +++ b/block/ll_rw_blk.c
> @@ -1330,6 +1330,8 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
>  				goto new_segment;
>  
>  			sg->length += nbytes;
> +			if (sg->length > 8192)
> +				printk("sg_len=%d\n", sg->length);
>  		} else {
>  new_segment:
>  			if (!sg)
> @@ -1349,6 +1351,8 @@ new_segment:
>  				sg = sg_next(sg);
>  			}
>  
> +			if (bvprv && (page_address(bvprv->bv_page) + bvprv->bv_len == page_address(bvec->bv_page)))
> +				printk("missed merge\n");
>  			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
>  			nsegs++;
>  		}
> 
..

Yeah, the first part is similar to my own hack.

For testing, try "dd if=/dev/sda of=/dev/null bs=4096k".
That *really* should end up using contiguous pages on most systems.

I figured out the git thing, and am now building some in-between kernels to try.

Cheers

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Jens Axboe wrote:
> >>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>Jens Axboe wrote:
> >>>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>>Mark Lord wrote:
> >>>>>>Jens Axboe wrote:
> >>>>>>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>>>>>Matthew Wilcox wrote:
> >>>>>>>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>>>>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> >>>>>>>>>>64KB for libata,
> >>>>>>>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>>>>>>>Just a suspicion ... could this be slab vs slub?  ie check your 
> >>>>>>>>>configs
> >>>>>>>>>are the same / similar between the two kernels.
> >>>>>>>>..
> >>>>>>>>
> >>>>>>>>Mmmm.. a good thought, that one.
> >>>>>>>>But I just rechecked, and both have CONFIG_SLAB=y
> >>>>>>>>
> >>>>>>>>My guess is that something got changed around when Jens
> >>>>>>>>reworked the block layer for 2.6.24.
> >>>>>>>>I'm going to dig around in there now.
> >>>>>>>I didn't rework the block layer for 2.6.24 :-). The core block layer
> >>>>>>>changes since 2.6.23 are:
> >>>>>>>
> >>>>>>>- Support for empty barriers. Not a likely candidate.
> >>>>>>>- Shared tag queue fixes. Totally unlikely.
> >>>>>>>- sg chaining support. Not likely.
> >>>>>>>- The bio changes from Neil. Of the bunch, the most likely suspects 
> >>>>>>>in
> >>>>>>>this area, since it changes some of the code involved with merges and
> >>>>>>>blk_rq_map_sg().
> >>>>>>>- Lots of simple stuff, again very unlikely.
> >>>>>>>
> >>>>>>>Anyway, it sounds odd for this to be a block layer problem if you do 
> >>>>>>>see
> >>>>>>>occasional segments being merged. So it sounds more like the input 
> >>>>>>>data
> >>>>>>>having changed.
> >>>>>>>
> >>>>>>>Why not just bisect it?
> >>>>>>..
> >>>>>>
> >>>>>>Because the early 2.6.24 series failed to boot on this machine
> >>>>>>due to bugs in the block layer -- so the code that caused this 
> >>>>>>regression
> >>>>>>is probably in the stuff from before the kernels became usable here.
> >>>>>..
> >>>>>
> >>>>>That sounds more harsh than intended --> the earlier 2.6.24 kernels 
> >>>>>(up to
> >>>>>the first couple of -rc* ones failed here because of incompatibilities
> >>>>>between the block/bio changes and libata.
> >>>>>
> >>>>>That's better, I think! 
> >>>>No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> >>>>
> >>>>If I were you, I'd just start from the first -rc that booted for you. If
> >>>>THAT has the bug, then we'll think of something else. If you don't get
> >>>>anywhere, I can run some tests tomorrow and see if I can reproduce it
> >>>>here.
> >>>..
> >>>
> >>>I believe that *anyone* can reproduce it, since it's broken long before
> >>>the requests ever get to SCSI or libata.  Which also means that *anyone*
> >>>who wants to can bisect it, as well.
> >>>
> >>>I don't do "bisects".
> >>It was just a suggestion on how to narrow it down, do as you see fit.
> >>
> >>>But I will dig a bit more and see if I can find the culprit.
> >>Sure, I'll dig around as well.
> >
> >Just tried something simple. I only see one 12kb segment so far, so not
> >a lot by any stretch. I also DONT see any missed merges signs, so it
> >would appear that the pages in the request are simply not contigious
> >physically.
> >
> >diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> >index e30b1a4..1e34b6f 100644
> >--- a/block/ll_rw_blk.c
> >+++ b/block/ll_rw_blk.c
> >@@ -1330,6 +1330,8 @@ int blk_rq_map_sg(struct request_queue *q, struct 
> >request *rq,
> > 				goto new_segment;
> > 
> > 			sg->length += nbytes;
> >+			if (sg->length > 8192)
> >+				printk("sg_len=%d\n", sg->length);
> > 		} else {
> > new_segment:
> > 			if (!sg)
> >@@ -1349,6 +1351,8 @@ new_segment:
> > 				sg = sg_next(sg);
> > 			}
> > 
> >+			if (bvprv && (page_address(bvprv->bv_page) + 
> >bvprv->bv_len == page_address(bvec->bv_page)))
> >+				printk("missed merge\n");
> > 			sg_set_page(sg, bvec->bv_page, nbytes, 
> > 			bvec->bv_offset);
> > 			nsegs++;
> > 		}
> >
> ..
> 
> Yeah, the first part is similar to my own hack.
> 
> For testing, try "dd if=/dev/sda of=/dev/null bs=4096k".
> That *really* should end up using contiguous pages on most systems.
> 
> I figured out the git thing, and am now building some in-between kernels to 
> try.

OK, it's a vm issue, I have tens of thousand "backward" pages after a
boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
reverse. So it looks like that bug got reintroduced.

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
>> Jens Axboe wrote:
>>> On Thu, Dec 13 2007, Jens Axboe wrote:
>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>> Jens Axboe wrote:
>>>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>>>> Mark Lord wrote:
>>>>>>>> Jens Axboe wrote:
>>>>>>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>>>>>>> Matthew Wilcox wrote:
>>>>>>>>>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>>>>>>>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 
>>>>>>>>>>>> 64KB for libata,
>>>>>>>>>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>>>>>>>>>> Just a suspicion ... could this be slab vs slub?  ie check your 
>>>>>>>>>>> configs
>>>>>>>>>>> are the same / similar between the two kernels.
>>>>>>>>>> ..
>>>>>>>>>>
>>>>>>>>>> Mmmm.. a good thought, that one.
>>>>>>>>>> But I just rechecked, and both have CONFIG_SLAB=y
>>>>>>>>>>
>>>>>>>>>> My guess is that something got changed around when Jens
>>>>>>>>>> reworked the block layer for 2.6.24.
>>>>>>>>>> I'm going to dig around in there now.
>>>>>>>>> I didn't rework the block layer for 2.6.24 :-). The core block layer
>>>>>>>>> changes since 2.6.23 are:
>>>>>>>>>
>>>>>>>>> - Support for empty barriers. Not a likely candidate.
>>>>>>>>> - Shared tag queue fixes. Totally unlikely.
>>>>>>>>> - sg chaining support. Not likely.
>>>>>>>>> - The bio changes from Neil. Of the bunch, the most likely suspects 
>>>>>>>>> in
>>>>>>>>> this area, since it changes some of the code involved with merges and
>>>>>>>>> blk_rq_map_sg().
>>>>>>>>> - Lots of simple stuff, again very unlikely.
>>>>>>>>>
>>>>>>>>> Anyway, it sounds odd for this to be a block layer problem if you do 
>>>>>>>>> see
>>>>>>>>> occasional segments being merged. So it sounds more like the input 
>>>>>>>>> data
>>>>>>>>> having changed.
>>>>>>>>>
>>>>>>>>> Why not just bisect it?
>>>>>>>> ..
>>>>>>>>
>>>>>>>> Because the early 2.6.24 series failed to boot on this machine
>>>>>>>> due to bugs in the block layer -- so the code that caused this 
>>>>>>>> regression
>>>>>>>> is probably in the stuff from before the kernels became usable here.
>>>>>>> ..
>>>>>>>
>>>>>>> That sounds more harsh than intended --> the earlier 2.6.24 kernels 
>>>>>>> (up to
>>>>>>> the first couple of -rc* ones failed here because of incompatibilities
>>>>>>> between the block/bio changes and libata.
>>>>>>>
>>>>>>> That's better, I think! 
>>>>>> No worries, I didn't pick it up as harsh just as an odd conclusion :-)
>>>>>>
>>>>>> If I were you, I'd just start from the first -rc that booted for you. If
>>>>>> THAT has the bug, then we'll think of something else. If you don't get
>>>>>> anywhere, I can run some tests tomorrow and see if I can reproduce it
>>>>>> here.
>>>>> ..
>>>>>
>>>>> I believe that *anyone* can reproduce it, since it's broken long before
>>>>> the requests ever get to SCSI or libata.  Which also means that *anyone*
>>>>> who wants to can bisect it, as well.
>>>>>
>>>>> I don't do "bisects".
>>>> It was just a suggestion on how to narrow it down, do as you see fit.
>>>>
>>>>> But I will dig a bit more and see if I can find the culprit.
>>>> Sure, I'll dig around as well.
>>> Just tried something simple. I only see one 12kb segment so far, so not
>>> a lot by any stretch. I also DONT see any missed merges signs, so it
>>> would appear that the pages in the request are simply not contigious
>>> physically.
>>>
>>> diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
>>> index e30b1a4..1e34b6f 100644
>>> --- a/block/ll_rw_blk.c
>>> +++ b/block/ll_rw_blk.c
>>> @@ -1330,6 +1330,8 @@ int blk_rq_map_sg(struct request_queue *q, struct 
>>> request *rq,
>>> 				goto new_segment;
>>>
>>> 			sg->length += nbytes;
>>> +			if (sg->length > 8192)
>>> +				printk("sg_len=%d\n", sg->length);
>>> 		} else {
>>> new_segment:
>>> 			if (!sg)
>>> @@ -1349,6 +1351,8 @@ new_segment:
>>> 				sg = sg_next(sg);
>>> 			}
>>>
>>> +			if (bvprv && (page_address(bvprv->bv_page) + 
>>> bvprv->bv_len == page_address(bvec->bv_page)))
>>> +				printk("missed merge\n");
>>> 			sg_set_page(sg, bvec->bv_page, nbytes, 
>>> 			bvec->bv_offset);
>>> 			nsegs++;
>>> 		}
>>>
>> ..
>>
>> Yeah, the first part is similar to my own hack.
>>
>> For testing, try "dd if=/dev/sda of=/dev/null bs=4096k".
>> That *really* should end up using contiguous pages on most systems.
>>
>> I figured out the git thing, and am now building some in-between kernels to 
>> try.
> 
> OK, it's a vm issue, I have tens of thousand "backward" pages after a
> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> reverse. So it looks like that bug got reintroduced.
...

Mmm.. shouldn't one of the front- or back- merge logics work for either order?

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mark Lord wrote:
> Jens Axboe wrote:
..
>> OK, it's a vm issue, I have tens of thousand "backward" pages after a
>> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
>> reverse. So it looks like that bug got reintroduced.
> ...
> 
> Mmm.. shouldn't one of the front- or back- merge logics work for either order?
..

Belay that thought.  I'm slowly remembering how this is supposed to work now.  :)

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Mark Lord wrote:
> >>Jens Axboe wrote:
> >>>On Thu, Dec 13 2007, Jens Axboe wrote:
> >>>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>>Jens Axboe wrote:
> >>>>>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>>>>Mark Lord wrote:
> >>>>>>>>Jens Axboe wrote:
> >>>>>>>>>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>>>>>>>>Matthew Wilcox wrote:
> >>>>>>>>>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>>>>>>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> >>>>>>>>>>>>64KB for libata,
> >>>>>>>>>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>>>>>>>>>Just a suspicion ... could this be slab vs slub?  ie check your 
> >>>>>>>>>>>configs
> >>>>>>>>>>>are the same / similar between the two kernels.
> >>>>>>>>>>..
> >>>>>>>>>>
> >>>>>>>>>>Mmmm.. a good thought, that one.
> >>>>>>>>>>But I just rechecked, and both have CONFIG_SLAB=y
> >>>>>>>>>>
> >>>>>>>>>>My guess is that something got changed around when Jens
> >>>>>>>>>>reworked the block layer for 2.6.24.
> >>>>>>>>>>I'm going to dig around in there now.
> >>>>>>>>>I didn't rework the block layer for 2.6.24 :-). The core block 
> >>>>>>>>>layer
> >>>>>>>>>changes since 2.6.23 are:
> >>>>>>>>>
> >>>>>>>>>- Support for empty barriers. Not a likely candidate.
> >>>>>>>>>- Shared tag queue fixes. Totally unlikely.
> >>>>>>>>>- sg chaining support. Not likely.
> >>>>>>>>>- The bio changes from Neil. Of the bunch, the most likely 
> >>>>>>>>>suspects in
> >>>>>>>>>this area, since it changes some of the code involved with merges 
> >>>>>>>>>and
> >>>>>>>>>blk_rq_map_sg().
> >>>>>>>>>- Lots of simple stuff, again very unlikely.
> >>>>>>>>>
> >>>>>>>>>Anyway, it sounds odd for this to be a block layer problem if you 
> >>>>>>>>>do see
> >>>>>>>>>occasional segments being merged. So it sounds more like the input 
> >>>>>>>>>data
> >>>>>>>>>having changed.
> >>>>>>>>>
> >>>>>>>>>Why not just bisect it?
> >>>>>>>>..
> >>>>>>>>
> >>>>>>>>Because the early 2.6.24 series failed to boot on this machine
> >>>>>>>>due to bugs in the block layer -- so the code that caused this 
> >>>>>>>>regression
> >>>>>>>>is probably in the stuff from before the kernels became usable here.
> >>>>>>>..
> >>>>>>>
> >>>>>>>That sounds more harsh than intended --> the earlier 2.6.24 kernels 
> >>>>>>>(up to
> >>>>>>>the first couple of -rc* ones failed here because of 
> >>>>>>>incompatibilities
> >>>>>>>between the block/bio changes and libata.
> >>>>>>>
> >>>>>>>That's better, I think! 
> >>>>>>No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> >>>>>>
> >>>>>>If I were you, I'd just start from the first -rc that booted for you. 
> >>>>>>If
> >>>>>>THAT has the bug, then we'll think of something else. If you don't get
> >>>>>>anywhere, I can run some tests tomorrow and see if I can reproduce it
> >>>>>>here.
> >>>>>..
> >>>>>
> >>>>>I believe that *anyone* can reproduce it, since it's broken long before
> >>>>>the requests ever get to SCSI or libata.  Which also means that 
> >>>>>*anyone*
> >>>>>who wants to can bisect it, as well.
> >>>>>
> >>>>>I don't do "bisects".
> >>>>It was just a suggestion on how to narrow it down, do as you see fit.
> >>>>
> >>>>>But I will dig a bit more and see if I can find the culprit.
> >>>>Sure, I'll dig around as well.
> >>>Just tried something simple. I only see one 12kb segment so far, so not
> >>>a lot by any stretch. I also DONT see any missed merges signs, so it
> >>>would appear that the pages in the request are simply not contigious
> >>>physically.
> >>>
> >>>diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> >>>index e30b1a4..1e34b6f 100644
> >>>--- a/block/ll_rw_blk.c
> >>>+++ b/block/ll_rw_blk.c
> >>>@@ -1330,6 +1330,8 @@ int blk_rq_map_sg(struct request_queue *q, struct 
> >>>request *rq,
> >>>				goto new_segment;
> >>>
> >>>			sg->length += nbytes;
> >>>+			if (sg->length > 8192)
> >>>+				printk("sg_len=%d\n", sg->length);
> >>>		} else {
> >>>new_segment:
> >>>			if (!sg)
> >>>@@ -1349,6 +1351,8 @@ new_segment:
> >>>				sg = sg_next(sg);
> >>>			}
> >>>
> >>>+			if (bvprv && (page_address(bvprv->bv_page) + 
> >>>bvprv->bv_len == page_address(bvec->bv_page)))
> >>>+				printk("missed merge\n");
> >>>			sg_set_page(sg, bvec->bv_page, nbytes, 
> >>>			bvec->bv_offset);
> >>>			nsegs++;
> >>>		}
> >>>
> >>..
> >>
> >>Yeah, the first part is similar to my own hack.
> >>
> >>For testing, try "dd if=/dev/sda of=/dev/null bs=4096k".
> >>That *really* should end up using contiguous pages on most systems.
> >>
> >>I figured out the git thing, and am now building some in-between kernels 
> >>to try.
> >
> >OK, it's a vm issue, I have tens of thousand "backward" pages after a
> >boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> >reverse. So it looks like that bug got reintroduced.
> ...
> 
> Mmm.. shouldn't one of the front- or back- merge logics work for either 
> order?

I think you are misunderstanding the merging. The front/back bits are
for contig on disk, this is sg segment merging. We can only join pieces
that are contig in memory, otherwise the result would not be pretty :-)

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Andrew Morton]

On Thu, 13 Dec 2007 21:09:59 +0100
Jens Axboe <jens.axboe@oracle.com> wrote:

>
> OK, it's a vm issue,

cc linux-mm and probable culprit.

>  I have tens of thousand "backward" pages after a
> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> reverse. So it looks like that bug got reintroduced.

Bill Irwin fixed this a couple of years back: changed the page allocator so
that it mostly hands out pages in ascending physical-address order.

I guess we broke that, quite possibly in Mel's page allocator rework.

It would help if you could provide us with a simple recipe for
demonstrating this problem, please.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[James Bottomley]

On Thu, 2007-12-13 at 14:02 -0800, Andrew Morton wrote:
> On Thu, 13 Dec 2007 21:09:59 +0100
> Jens Axboe <jens.axboe@oracle.com> wrote:
> 
> >
> > OK, it's a vm issue,
> 
> cc linux-mm and probable culprit.
> 
> >  I have tens of thousand "backward" pages after a
> > boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> > reverse. So it looks like that bug got reintroduced.
> 
> Bill Irwin fixed this a couple of years back: changed the page allocator so
> that it mostly hands out pages in ascending physical-address order.
> 
> I guess we broke that, quite possibly in Mel's page allocator rework.
> 
> It would help if you could provide us with a simple recipe for
> demonstrating this problem, please.

The simple way seems to be to malloc a large area, touch every page and
then look at the physical pages assigned ... they now mostly seem to be
descending in physical address.

James

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Andrew Morton]

On Thu, 13 Dec 2007 17:15:06 -0500
James Bottomley <James.Bottomley@HansenPartnership.com> wrote:

> 
> On Thu, 2007-12-13 at 14:02 -0800, Andrew Morton wrote:
> > On Thu, 13 Dec 2007 21:09:59 +0100
> > Jens Axboe <jens.axboe@oracle.com> wrote:
> > 
> > >
> > > OK, it's a vm issue,
> > 
> > cc linux-mm and probable culprit.
> > 
> > >  I have tens of thousand "backward" pages after a
> > > boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> > > reverse. So it looks like that bug got reintroduced.
> > 
> > Bill Irwin fixed this a couple of years back: changed the page allocator so
> > that it mostly hands out pages in ascending physical-address order.
> > 
> > I guess we broke that, quite possibly in Mel's page allocator rework.
> > 
> > It would help if you could provide us with a simple recipe for
> > demonstrating this problem, please.
> 
> The simple way seems to be to malloc a large area, touch every page and
> then look at the physical pages assigned ... they now mostly seem to be
> descending in physical address.
> 

OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Andrew Morton wrote:
> On Thu, 13 Dec 2007 17:15:06 -0500
> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> 
>> On Thu, 2007-12-13 at 14:02 -0800, Andrew Morton wrote:
>>> On Thu, 13 Dec 2007 21:09:59 +0100
>>> Jens Axboe <jens.axboe@oracle.com> wrote:
>>>
>>>> OK, it's a vm issue,
>>> cc linux-mm and probable culprit.
>>>
>>>>  I have tens of thousand "backward" pages after a
>>>> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
>>>> reverse. So it looks like that bug got reintroduced.
>>> Bill Irwin fixed this a couple of years back: changed the page allocator so
>>> that it mostly hands out pages in ascending physical-address order.
>>>
>>> I guess we broke that, quite possibly in Mel's page allocator rework.
>>>
>>> It would help if you could provide us with a simple recipe for
>>> demonstrating this problem, please.
>> The simple way seems to be to malloc a large area, touch every page and
>> then look at the physical pages assigned ... they now mostly seem to be
>> descending in physical address.
>>
> 
> OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
..

I'm actually running the treadmill right now (have been for many hours, actually,
to bisect it to a specific commit.

Thought I was almost done, and then noticed that git-bisect doesn't keep
the Makefile VERSION lines the same, so I was actually running the wrong
kernel after the first few times.. duh.

Wrote a script to fix it now.

-ml

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mark Lord wrote:
> Andrew Morton wrote:
>> On Thu, 13 Dec 2007 17:15:06 -0500
>> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>
>>> On Thu, 2007-12-13 at 14:02 -0800, Andrew Morton wrote:
>>>> On Thu, 13 Dec 2007 21:09:59 +0100
>>>> Jens Axboe <jens.axboe@oracle.com> wrote:
>>>>
>>>>> OK, it's a vm issue,
>>>> cc linux-mm and probable culprit.
>>>>
>>>>>  I have tens of thousand "backward" pages after a
>>>>> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
>>>>> reverse. So it looks like that bug got reintroduced.
>>>> Bill Irwin fixed this a couple of years back: changed the page 
>>>> allocator so
>>>> that it mostly hands out pages in ascending physical-address order.
>>>>
>>>> I guess we broke that, quite possibly in Mel's page allocator rework.
>>>>
>>>> It would help if you could provide us with a simple recipe for
>>>> demonstrating this problem, please.
>>> The simple way seems to be to malloc a large area, touch every page and
>>> then look at the physical pages assigned ... they now mostly seem to be
>>> descending in physical address.
>>>
>>
>> OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
> ..
> 
> I'm actually running the treadmill right now (have been for many hours, 
> actually,
> to bisect it to a specific commit.
> 
> Thought I was almost done, and then noticed that git-bisect doesn't keep
> the Makefile VERSION lines the same, so I was actually running the wrong
> kernel after the first few times.. duh.
> 
> Wrote a script to fix it now.
..

Well, that was a waste of three hours.

Somebody else can try it now.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mark Lord wrote:
> Mark Lord wrote:
>> Andrew Morton wrote:
>>> On Thu, 13 Dec 2007 17:15:06 -0500
>>> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>>
>>>> On Thu, 2007-12-13 at 14:02 -0800, Andrew Morton wrote:
>>>>> On Thu, 13 Dec 2007 21:09:59 +0100
>>>>> Jens Axboe <jens.axboe@oracle.com> wrote:
>>>>>
>>>>>> OK, it's a vm issue,
>>>>> cc linux-mm and probable culprit.
>>>>>
>>>>>>  I have tens of thousand "backward" pages after a
>>>>>> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
>>>>>> reverse. So it looks like that bug got reintroduced.
>>>>> Bill Irwin fixed this a couple of years back: changed the page 
>>>>> allocator so
>>>>> that it mostly hands out pages in ascending physical-address order.
>>>>>
>>>>> I guess we broke that, quite possibly in Mel's page allocator rework.
>>>>>
>>>>> It would help if you could provide us with a simple recipe for
>>>>> demonstrating this problem, please.
>>>> The simple way seems to be to malloc a large area, touch every page and
>>>> then look at the physical pages assigned ... they now mostly seem to be
>>>> descending in physical address.
>>>>
>>>
>>> OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
>> ..
>>
>> I'm actually running the treadmill right now (have been for many 
>> hours, actually,
>> to bisect it to a specific commit.
>>
>> Thought I was almost done, and then noticed that git-bisect doesn't keep
>> the Makefile VERSION lines the same, so I was actually running the wrong
>> kernel after the first few times.. duh.
>>
>> Wrote a script to fix it now.
> ..
> 
> Well, that was a waste of three hours.
..

Ahh.. it seems to be sensitive to one/both of these:

CONFIG_HIGHMEM64G=y with 4GB RAM:  not so bad, frequently does 20KB - 48KB segments.
CONFIG_HIGHMEM4G=y  with 2GB RAM:  very severe, rarely does more than 8KB segments.
CONFIG_HIGHMEM4G=y  with 3GB RAM:  very severe, rarely does more than 8KB segments.

So if you want to reproduce this on a large memory machine, use "mem=2GB" for starters.

Still testing..

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mark Lord wrote:
> Mark Lord wrote:
>> Mark Lord wrote:
>>> Andrew Morton wrote:
>>>> On Thu, 13 Dec 2007 17:15:06 -0500
>>>> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>>>
>>>>> On Thu, 2007-12-13 at 14:02 -0800, Andrew Morton wrote:
>>>>>> On Thu, 13 Dec 2007 21:09:59 +0100
>>>>>> Jens Axboe <jens.axboe@oracle.com> wrote:
>>>>>>
>>>>>>> OK, it's a vm issue,
>>>>>> cc linux-mm and probable culprit.
>>>>>>
>>>>>>>  I have tens of thousand "backward" pages after a
>>>>>>> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
>>>>>>> reverse. So it looks like that bug got reintroduced.
>>>>>> Bill Irwin fixed this a couple of years back: changed the page 
>>>>>> allocator so
>>>>>> that it mostly hands out pages in ascending physical-address order.
>>>>>>
>>>>>> I guess we broke that, quite possibly in Mel's page allocator rework.
>>>>>>
>>>>>> It would help if you could provide us with a simple recipe for
>>>>>> demonstrating this problem, please.
>>>>> The simple way seems to be to malloc a large area, touch every page 
>>>>> and
>>>>> then look at the physical pages assigned ... they now mostly seem 
>>>>> to be
>>>>> descending in physical address.
>>>>>
>>>>
>>>> OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
>>> ..
>>>
>>> I'm actually running the treadmill right now (have been for many 
>>> hours, actually,
>>> to bisect it to a specific commit.
>>>
>>> Thought I was almost done, and then noticed that git-bisect doesn't keep
>>> the Makefile VERSION lines the same, so I was actually running the wrong
>>> kernel after the first few times.. duh.
>>>
>>> Wrote a script to fix it now.
>> ..
>>
>> Well, that was a waste of three hours.
> ..
> 
> Ahh.. it seems to be sensitive to one/both of these:
> 
> CONFIG_HIGHMEM64G=y with 4GB RAM:  not so bad, frequently does 20KB - 
> 48KB segments.
> CONFIG_HIGHMEM4G=y  with 2GB RAM:  very severe, rarely does more than 
> 8KB segments.
> CONFIG_HIGHMEM4G=y  with 3GB RAM:  very severe, rarely does more than 
> 8KB segments.
> 
> So if you want to reproduce this on a large memory machine, use 
> "mem=2GB" for starters.
..

Here's the commit that causes the regression:

535131e6925b4a95f321148ad7293f496e0e58d7 Choose pages from the per-cpu list based on migration type


From: Mel Gorman <mel@csn.ul.ie>
Date: Tue, 16 Oct 2007 08:25:49 +0000 (-0700)
Subject: Choose pages from the per-cpu list based on migration type
X-Git-Tag: v2.6.24-rc1~1141
X-Git-Url: http://git.kernel.org/?p=linux%2Fkernel%2Fgit%2Ftorvalds%2Flinux-2.6.git;a=commitdiff_plain;h=535131e6925b4a95f321148ad7293f496e0e58d7;hp=b2a0ac8875a0a3b9f0739b60526f8c5977d2200f

Choose pages from the per-cpu list based on migration type

The freelists for each migrate type can slowly become polluted due to the
per-cpu list.  Consider what happens when the following happens

1. A 2^(MAX_ORDER-1) list is reserved for __GFP_MOVABLE pages
2. An order-0 page is allocated from the newly reserved block
3. The page is freed and placed on the per-cpu list
4. alloc_page() is called with GFP_KERNEL as the gfp_mask
5. The per-cpu list is used to satisfy the allocation

This results in a kernel page is in the middle of a migratable region. This
patch prevents this leak occuring by storing the MIGRATE_ type of the page in
page->private. On allocate, a page will only be returned of the desired type,
else more pages will be allocated. This may temporarily allow a per-cpu list
to go over the pcp->high limit but it'll be corrected on the next free. Care
is taken to preserve the hotness of pages recently freed.

The additional code is not measurably slower for the workloads we've tested.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d54ecf4..e3e726b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -760,7 +760,8 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 		struct page *page = __rmqueue(zone, order, migratetype);
 		if (unlikely(page == NULL))
 			break;
-		list_add_tail(&page->lru, list);
+		list_add(&page->lru, list);
+		set_page_private(page, migratetype);
 	}
 	spin_unlock(&zone->lock);
 	return i;
@@ -887,6 +888,7 @@ static void fastcall free_hot_cold_page(struct page *page, int cold)
 	local_irq_save(flags);
 	__count_vm_event(PGFREE);
 	list_add(&page->lru, &pcp->list);
+	set_page_private(page, get_pageblock_migratetype(page));
 	pcp->count++;
 	if (pcp->count >= pcp->high) {
 		free_pages_bulk(zone, pcp->batch, &pcp->list, 0);
@@ -951,9 +953,27 @@ again:
 			if (unlikely(!pcp->count))
 				goto failed;
 		}
-		page = list_entry(pcp->list.next, struct page, lru);
-		list_del(&page->lru);
-		pcp->count--;
+		/* Find a page of the appropriate migrate type */
+		list_for_each_entry(page, &pcp->list, lru) {
+			if (page_private(page) == migratetype) {
+				list_del(&page->lru);
+				pcp->count--;
+				break;
+			}
+		}
+
+		/*
+		 * Check if a page of the appropriate migrate type
+		 * was found. If not, allocate more to the pcp list
+		 */
+		if (&page->lru == &pcp->list) {
+			pcp->count += rmqueue_bulk(zone, 0,
+					pcp->batch, &pcp->list, migratetype);
+			page = list_entry(pcp->list.next, struct page, lru);
+			VM_BUG_ON(page_private(page) != migratetype);
+			list_del(&page->lru);
+			pcp->count--;
+		}
 	} else {
 		spin_lock_irqsave(&zone->lock, flags);
 		page = __rmqueue(zone, order, migratetype);

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Andrew Morton]

On Thu, 13 Dec 2007 19:30:00 -0500
Mark Lord <liml@rtr.ca> wrote:

> Here's the commit that causes the regression:
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -760,7 +760,8 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>  		struct page *page = __rmqueue(zone, order, migratetype);
>  		if (unlikely(page == NULL))
>  			break;
> -		list_add_tail(&page->lru, list);
> +		list_add(&page->lru, list);

well that looks fishy.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Andrew Morton wrote:
> On Thu, 13 Dec 2007 19:30:00 -0500
> Mark Lord <liml@rtr.ca> wrote:
> 
>> Here's the commit that causes the regression:
>>
>> ...
>>
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -760,7 +760,8 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>>  		struct page *page = __rmqueue(zone, order, migratetype);
>>  		if (unlikely(page == NULL))
>>  			break;
>> -		list_add_tail(&page->lru, list);
>> +		list_add(&page->lru, list);
> 
> well that looks fishy.
..

Yeah.  I missed that, and instead just posted a patch
to search the list in reverse order, which seems to work for me.

I'll try just reversing that line above here now.. gimme 5 minutes or so.

Cheers

[PATCH] fix page_alloc for larger I/O segments (improved)

[Mark Lord]

"Improved version", more similar to the 2.6.23 code:

Fix page allocator to give better chance of larger contiguous segments (again).

Signed-off-by: Mark Lord <mlord@pobox.com
---

--- old/mm/page_alloc.c	2007-12-13 19:25:15.000000000 -0500
+++ linux-2.6/mm/page_alloc.c	2007-12-13 19:43:07.000000000 -0500
@@ -760,7 +760,7 @@
 		struct page *page = __rmqueue(zone, order, migratetype);
 		if (unlikely(page == NULL))
 			break;
-		list_add(&page->lru, list);
+		list_add_tail(&page->lru, list);
 		set_page_private(page, migratetype);
 	}
 	spin_unlock(&zone->lock);

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[James Bottomley]

On Thu, 2007-12-13 at 19:46 -0500, Mark Lord wrote:
> "Improved version", more similar to the 2.6.23 code:
> 
> Fix page allocator to give better chance of larger contiguous segments (again).
> 
> Signed-off-by: Mark Lord <mlord@pobox.com
> ---
> 
> --- old/mm/page_alloc.c	2007-12-13 19:25:15.000000000 -0500
> +++ linux-2.6/mm/page_alloc.c	2007-12-13 19:43:07.000000000 -0500
> @@ -760,7 +760,7 @@
>  		struct page *page = __rmqueue(zone, order, migratetype);
>  		if (unlikely(page == NULL))
>  			break;
> -		list_add(&page->lru, list);
> +		list_add_tail(&page->lru, list);

Could we put a big comment above this explaining to the would be vm
tweakers why this has to be a list_add_tail, so we don't end up back in
this position after another two years?

James

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Andrew Morton]

On Thu, 13 Dec 2007 19:57:29 -0500
James Bottomley <James.Bottomley@HansenPartnership.com> wrote:

> 
> On Thu, 2007-12-13 at 19:46 -0500, Mark Lord wrote:
> > "Improved version", more similar to the 2.6.23 code:
> > 
> > Fix page allocator to give better chance of larger contiguous segments (again).
> > 
> > Signed-off-by: Mark Lord <mlord@pobox.com
> > ---
> > 
> > --- old/mm/page_alloc.c	2007-12-13 19:25:15.000000000 -0500
> > +++ linux-2.6/mm/page_alloc.c	2007-12-13 19:43:07.000000000 -0500
> > @@ -760,7 +760,7 @@
> >  		struct page *page = __rmqueue(zone, order, migratetype);
> >  		if (unlikely(page == NULL))
> >  			break;
> > -		list_add(&page->lru, list);
> > +		list_add_tail(&page->lru, list);
> 
> Could we put a big comment above this explaining to the would be vm
> tweakers why this has to be a list_add_tail, so we don't end up back in
> this position after another two years?
> 

Already done ;)

--- a/mm/page_alloc.c~fix-page_alloc-for-larger-i-o-segments-fix
+++ a/mm/page_alloc.c
@@ -847,6 +847,10 @@ static int rmqueue_bulk(struct zone *zon
 		struct page *page = __rmqueue(zone, order, migratetype);
 		if (unlikely(page == NULL))
 			break;
+		/*
+		 * Doing a list_add_tail() here helps us to hand out pages in
+		 * ascending physical-address order.
+		 */
 		list_add_tail(&page->lru, list);
 		set_page_private(page, migratetype);
 	}
_

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Mark Lord]

Andrew Morton wrote:
> On Thu, 13 Dec 2007 19:57:29 -0500
> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> 
>> On Thu, 2007-12-13 at 19:46 -0500, Mark Lord wrote:
>>> "Improved version", more similar to the 2.6.23 code:
>>>
>>> Fix page allocator to give better chance of larger contiguous segments (again).
>>>
>>> Signed-off-by: Mark Lord <mlord@pobox.com
>>> ---
>>>
>>> --- old/mm/page_alloc.c	2007-12-13 19:25:15.000000000 -0500
>>> +++ linux-2.6/mm/page_alloc.c	2007-12-13 19:43:07.000000000 -0500
>>> @@ -760,7 +760,7 @@
>>>  		struct page *page = __rmqueue(zone, order, migratetype);
>>>  		if (unlikely(page == NULL))
>>>  			break;
>>> -		list_add(&page->lru, list);
>>> +		list_add_tail(&page->lru, list);
>> Could we put a big comment above this explaining to the would be vm
>> tweakers why this has to be a list_add_tail, so we don't end up back in
>> this position after another two years?
>>
> 
> Already done ;)
..

I thought of the comment as I rushed off for dinner.
Thanks, Andrew!

> --- a/mm/page_alloc.c~fix-page_alloc-for-larger-i-o-segments-fix
> +++ a/mm/page_alloc.c
> @@ -847,6 +847,10 @@ static int rmqueue_bulk(struct zone *zon
>  		struct page *page = __rmqueue(zone, order, migratetype);
>  		if (unlikely(page == NULL))
>  			break;
> +		/*
> +		 * Doing a list_add_tail() here helps us to hand out pages in
> +		 * ascending physical-address order.
> +		 */
>  		list_add_tail(&page->lru, list);
>  		set_page_private(page, migratetype);
>  	}
> _

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Mel Gorman]

On (13/12/07 19:46), Mark Lord didst pronounce:
> 
> "Improved version", more similar to the 2.6.23 code:
> 
> Fix page allocator to give better chance of larger contiguous segments 
> (again).
> 
> Signed-off-by: Mark Lord <mlord@pobox.com

Regrettably this interferes with anti-fragmentation because the "next" page
on the list on return from rmqueue_bulk is not guaranteed to be of the right
mobility type. I fixed it as an additional patch but it adds additional cost
that should not be necessary and it's visible in microbenchmark results on
at least one machine.

The following patch should fix the page ordering problem without incurring an
additional cost or interfering with anti-fragmentation. However, I haven't
anything in place yet to verify that the physical page ordering is correct
but it makes sense. Can you verify it fixes the problem please?

It'll still be some time before I have a full set of performance results
but initially at least, this fix seems to avoid any impact.

======
Subject: Fix page allocation for larger I/O segments

In some cases the IO subsystem is able to merge requests if the pages are
adjacent in physical memory. This was achieved in the allocator by having
expand() return pages in physically contiguous order in situations were
a large buddy was split. However, list-based anti-fragmentation changed
the order pages were returned in to avoid searching in buffered_rmqueue()
for a page of the appropriate migrate type.

This patch restores behaviour of rmqueue_bulk() preserving the physical order
of pages returned by the allocator without incurring increased search costs for
anti-fragmentation.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
--- 
 page_alloc.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.24-rc5-clean/mm/page_alloc.c linux-2.6.24-rc5-giveback-physorder-listmove/mm/page_alloc.c
--- linux-2.6.24-rc5-clean/mm/page_alloc.c	2007-12-14 11:55:13.000000000 +0000
+++ linux-2.6.24-rc5-giveback-physorder-listmove/mm/page_alloc.c	2007-12-14 15:33:12.000000000 +0000
@@ -847,8 +847,19 @@ static int rmqueue_bulk(struct zone *zon
 		struct page *page = __rmqueue(zone, order, migratetype);
 		if (unlikely(page == NULL))
 			break;
+
+		/*
+		 * Split buddy pages returned by expand() are received here
+		 * in physical page order. The page is added to the callers and
+		 * list and the list head then moves forward. From the callers
+		 * perspective, the linked list is ordered by page number in
+		 * some conditions. This is useful for IO devices that can
+		 * merge IO requests if the physical pages are ordered
+		 * properly.
+		 */
 		list_add(&page->lru, list);
 		set_page_private(page, migratetype);
+		list = &page->lru;
 	}
 	spin_unlock(&zone->lock);
 	return i;

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Mark Lord]

Mel Gorman wrote:
> On (13/12/07 19:46), Mark Lord didst pronounce:
>> "Improved version", more similar to the 2.6.23 code:
>>
>> Fix page allocator to give better chance of larger contiguous segments 
>> (again).
>>
>> Signed-off-by: Mark Lord <mlord@pobox.com
> 
> Regrettably this interferes with anti-fragmentation because the "next" page
> on the list on return from rmqueue_bulk is not guaranteed to be of the right
> mobility type. I fixed it as an additional patch but it adds additional cost
> that should not be necessary and it's visible in microbenchmark results on
> at least one machine.
> 
> The following patch should fix the page ordering problem without incurring an
> additional cost or interfering with anti-fragmentation. However, I haven't
> anything in place yet to verify that the physical page ordering is correct
> but it makes sense. Can you verify it fixes the problem please?
> 
> It'll still be some time before I have a full set of performance results
> but initially at least, this fix seems to avoid any impact.
> 
> ======
> Subject: Fix page allocation for larger I/O segments
> 
> In some cases the IO subsystem is able to merge requests if the pages are
> adjacent in physical memory. This was achieved in the allocator by having
> expand() return pages in physically contiguous order in situations were
> a large buddy was split. However, list-based anti-fragmentation changed
> the order pages were returned in to avoid searching in buffered_rmqueue()
> for a page of the appropriate migrate type.
> 
> This patch restores behaviour of rmqueue_bulk() preserving the physical order
> of pages returned by the allocator without incurring increased search costs for
> anti-fragmentation.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> --- 
>  page_alloc.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.24-rc5-clean/mm/page_alloc.c linux-2.6.24-rc5-giveback-physorder-listmove/mm/page_alloc.c
> --- linux-2.6.24-rc5-clean/mm/page_alloc.c	2007-12-14 11:55:13.000000000 +0000
> +++ linux-2.6.24-rc5-giveback-physorder-listmove/mm/page_alloc.c	2007-12-14 15:33:12.000000000 +0000
> @@ -847,8 +847,19 @@ static int rmqueue_bulk(struct zone *zon
>  		struct page *page = __rmqueue(zone, order, migratetype);
>  		if (unlikely(page == NULL))
>  			break;
> +
> +		/*
> +		 * Split buddy pages returned by expand() are received here
> +		 * in physical page order. The page is added to the callers and
> +		 * list and the list head then moves forward. From the callers
> +		 * perspective, the linked list is ordered by page number in
> +		 * some conditions. This is useful for IO devices that can
> +		 * merge IO requests if the physical pages are ordered
> +		 * properly.
> +		 */
>  		list_add(&page->lru, list);
>  		set_page_private(page, migratetype);
> +		list = &page->lru;
>  	}
>  	spin_unlock(&zone->lock);
>  	return i;
..

That (also) works for me here, regularly generating 64KB I/O segments with SLAB.

Cheers

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Mel Gorman]

On (14/12/07 13:07), Mark Lord didst pronounce:
> <SNIP>
> 
> That (also) works for me here, regularly generating 64KB I/O segments with 
> SLAB.
> 

Brilliant. Thanks a lot Mark.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Matthew Wilcox]

On Fri, Dec 14, 2007 at 05:42:37PM +0000, Mel Gorman wrote:
> Regrettably this interferes with anti-fragmentation because the "next" page
> on the list on return from rmqueue_bulk is not guaranteed to be of the right
> mobility type. I fixed it as an additional patch but it adds additional cost
> that should not be necessary and it's visible in microbenchmark results on
> at least one machine.

Is this patch to be preferred to the one Andrew Morton posted to do
list_for_each_entry_reverse?

I'll send it to our DB team to see if this improves our numbers at all.

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Mark Lord]

Matthew Wilcox wrote:
> On Fri, Dec 14, 2007 at 05:42:37PM +0000, Mel Gorman wrote:
>> Regrettably this interferes with anti-fragmentation because the "next" page
>> on the list on return from rmqueue_bulk is not guaranteed to be of the right
>> mobility type. I fixed it as an additional patch but it adds additional cost
>> that should not be necessary and it's visible in microbenchmark results on
>> at least one machine.
> 
> Is this patch to be preferred to the one Andrew Morton posted to do
> list_for_each_entry_reverse?
..

This patch replaces my earlier patch that Andrew has:

-               list_add(&page->lru, list);
+               list_add_tail(&page->lru, list);

Which, in turn, replaced the even-earlier list_for_each_entry_reverse patch.

-ml

Re: [PATCH] fix page_alloc for larger I/O segments (improved)

[Matthew Wilcox]

On Fri, Dec 14, 2007 at 11:13:40AM -0700, Matthew Wilcox wrote:
> I'll send it to our DB team to see if this improves our numbers at all.

It does, by approximately 0.67%.  This is about double the margin of
error, and a significant improvement.  Thanks!

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mark Lord wrote:
> Andrew Morton wrote:
>> On Thu, 13 Dec 2007 19:30:00 -0500
>> Mark Lord <liml@rtr.ca> wrote:
>>
>>> Here's the commit that causes the regression:
>>>
>>> ...
>>>
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -760,7 +760,8 @@ static int rmqueue_bulk(struct zone *zone, 
>>> unsigned int order,
>>>          struct page *page = __rmqueue(zone, order, migratetype);
>>>          if (unlikely(page == NULL))
>>>              break;
>>> -        list_add_tail(&page->lru, list);
>>> +        list_add(&page->lru, list);
>>
>> well that looks fishy.
> ..
> 
> Yeah.  I missed that, and instead just posted a patch
> to search the list in reverse order, which seems to work for me.
> 
> I'll try just reversing that line above here now.. gimme 5 minutes or so.
..

Yep, that works too.  Alternative "improved" patch now posted.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mel Gorman]

On (13/12/07 16:37), Andrew Morton didst pronounce:
> On Thu, 13 Dec 2007 19:30:00 -0500
> Mark Lord <liml@rtr.ca> wrote:
> 
> > Here's the commit that causes the regression:
> > 
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -760,7 +760,8 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
> >  		struct page *page = __rmqueue(zone, order, migratetype);
> >  		if (unlikely(page == NULL))
> >  			break;
> > -		list_add_tail(&page->lru, list);
> > +		list_add(&page->lru, list);
> 
> well that looks fishy.
> 

The reasoning behind the change was the first page encountered on the list
by the caller would have a matching migratetype. I failed to take into
account the physical ordering of pages returned. I'm setting up to run some
performance benchmarks of the candidate fix merged into the -mm tree to see
if the search shows up or not. I'm testing against 2.6.25-rc5 but it'll
take a few hours to complete.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Mel Gorman wrote:
> On (13/12/07 16:37), Andrew Morton didst pronounce:
>> On Thu, 13 Dec 2007 19:30:00 -0500
>> Mark Lord <liml@rtr.ca> wrote:
>>
>>> Here's the commit that causes the regression:
>>>
>>> ...
>>>
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -760,7 +760,8 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>>>  		struct page *page = __rmqueue(zone, order, migratetype);
>>>  		if (unlikely(page == NULL))
>>>  			break;
>>> -		list_add_tail(&page->lru, list);
>>> +		list_add(&page->lru, list);
>> well that looks fishy.
>>
> 
> The reasoning behind the change was the first page encountered on the list
> by the caller would have a matching migratetype. I failed to take into
> account the physical ordering of pages returned. I'm setting up to run some
> performance benchmarks of the candidate fix merged into the -mm tree to see
> if the search shows up or not. I'm testing against 2.6.25-rc5 but it'll
> take a few hours to complete.
..

Thanks, Mel.  This is all with CONFIG_SLAB=y, by the way.

Note that it did appear to behave better with CONFIG_SLUB=y when I accidently
used that .config on my 4GB machine here.  Physical segments of 4-10 pages
happended much more common than with CONFIG_SLAB=y on my 3GB machine
Slightly "apples and oranges" there, I know, but at least both were x86-32.  :)

So I would expect CONFIG_SLAB to be well off with this patch under most (all?)
conditions, but dunno about CONFIG_SLUB.

Cheers

[PATCH] fix page_alloc for larger I/O segments

[Mark Lord]

Mark Lord wrote:
> Mark Lord wrote:
>> Mark Lord wrote:
>>> Mark Lord wrote:
>>>> Andrew Morton wrote:
>>>>> On Thu, 13 Dec 2007 17:15:06 -0500
>>>>> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>>>>
>>>>>> On Thu, 2007-12-13 at 14:02 -0800, Andrew Morton wrote:
>>>>>>> On Thu, 13 Dec 2007 21:09:59 +0100
>>>>>>> Jens Axboe <jens.axboe@oracle.com> wrote:
>>>>>>>
>>>>>>>> OK, it's a vm issue,
>>>>>>> cc linux-mm and probable culprit.
>>>>>>>
>>>>>>>>  I have tens of thousand "backward" pages after a
>>>>>>>> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
>>>>>>>> reverse. So it looks like that bug got reintroduced.
>>>>>>> Bill Irwin fixed this a couple of years back: changed the page 
>>>>>>> allocator so
>>>>>>> that it mostly hands out pages in ascending physical-address order.
>>>>>>>
>>>>>>> I guess we broke that, quite possibly in Mel's page allocator 
>>>>>>> rework.
>>>>>>>
>>>>>>> It would help if you could provide us with a simple recipe for
>>>>>>> demonstrating this problem, please.
>>>>>> The simple way seems to be to malloc a large area, touch every 
>>>>>> page and
>>>>>> then look at the physical pages assigned ... they now mostly seem 
>>>>>> to be
>>>>>> descending in physical address.
>>>>>>
>>>>>
>>>>> OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
>>>> ..
>>>>
>>>> I'm actually running the treadmill right now (have been for many 
>>>> hours, actually,
>>>> to bisect it to a specific commit.
>>>>
>>>> Thought I was almost done, and then noticed that git-bisect doesn't 
>>>> keep
>>>> the Makefile VERSION lines the same, so I was actually running the 
>>>> wrong
>>>> kernel after the first few times.. duh.
>>>>
>>>> Wrote a script to fix it now.
>>> ..
>>>
>>> Well, that was a waste of three hours.
>> ..
>>
>> Ahh.. it seems to be sensitive to one/both of these:
>>
>> CONFIG_HIGHMEM64G=y with 4GB RAM:  not so bad, frequently does 20KB - 
>> 48KB segments.
>> CONFIG_HIGHMEM4G=y  with 2GB RAM:  very severe, rarely does more than 
>> 8KB segments.
>> CONFIG_HIGHMEM4G=y  with 3GB RAM:  very severe, rarely does more than 
>> 8KB segments.
>>
>> So if you want to reproduce this on a large memory machine, use 
>> "mem=2GB" for starters.
> ..
> 
> Here's the commit that causes the regression:
> 
> 535131e6925b4a95f321148ad7293f496e0e58d7 Choose pages from the per-cpu 
> list based on migration type
> 

And here is a patch that seems to fix it for me here:

* * * *

Fix page allocator to give better change of larger contiguous segments (again).

Signed-off-by: Mark Lord <mlord@pobox.com
---


--- old/mm/page_alloc.c.orig	2007-12-13 19:25:15.000000000 -0500
+++ linux-2.6/mm/page_alloc.c	2007-12-13 19:35:50.000000000 -0500
@@ -954,7 +954,7 @@
 				goto failed;
 		}
 		/* Find a page of the appropriate migrate type */
-		list_for_each_entry(page, &pcp->list, lru) {
+		list_for_each_entry_reverse(page, &pcp->list, lru) {
 			if (page_private(page) == migratetype) {
 				list_del(&page->lru);
 				pcp->count--;

Re: [PATCH] fix page_alloc for larger I/O segments

[Andrew Morton]

On Thu, 13 Dec 2007 19:40:09 -0500
Mark Lord <liml@rtr.ca> wrote:

> And here is a patch that seems to fix it for me here:
> 
> * * * *
> 
> Fix page allocator to give better change of larger contiguous segments (again).
> 
> Signed-off-by: Mark Lord <mlord@pobox.com
> ---
> 
> 
> --- old/mm/page_alloc.c.orig	2007-12-13 19:25:15.000000000 -0500
> +++ linux-2.6/mm/page_alloc.c	2007-12-13 19:35:50.000000000 -0500
> @@ -954,7 +954,7 @@
>  				goto failed;
>  		}
>  		/* Find a page of the appropriate migrate type */
> -		list_for_each_entry(page, &pcp->list, lru) {
> +		list_for_each_entry_reverse(page, &pcp->list, lru) {
>  			if (page_private(page) == migratetype) {
>  				list_del(&page->lru);
>  				pcp->count--;

- needs help to make it apply to mainline

- needs a comment, methinks...


--- a/mm/page_alloc.c~fix-page-allocator-to-give-better-chance-of-larger-contiguous-segments-again
+++ a/mm/page_alloc.c
@@ -1060,8 +1060,12 @@ again:
 				goto failed;
 		}
 
-		/* Find a page of the appropriate migrate type */
-		list_for_each_entry(page, &pcp->list, lru)
+		/*
+		 * Find a page of the appropriate migrate type.  Doing a
+		 * reverse-order search here helps us to hand out pages in
+		 * ascending physical-address order.
+		 */
+		list_for_each_entry_reverse(page, &pcp->list, lru)
 			if (page_private(page) == migratetype)
 				break;
 
_

Re: [PATCH] fix page_alloc for larger I/O segments

[Matthew Wilcox]

On Thu, Dec 13, 2007 at 05:03:08PM -0800, Andrew Morton wrote:
> +		/*
> +		 * Find a page of the appropriate migrate type.  Doing a
> +		 * reverse-order search here helps us to hand out pages in
> +		 * ascending physical-address order.
> +		 */
> +		list_for_each_entry_reverse(page, &pcp->list, lru)

It's not obvious why ascending physical order is a good thing.  How
about:

+		/*
+		 * Find a page of the appropriate migrate type.  Doing a
+		 * reverse-order search here helps us to hand out pages in
+		 * ascending physical-address order, which improves our
+		 * chances of coalescing scatter-gather pages.
+		 */

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mel Gorman]

On (13/12/07 14:29), Andrew Morton didst pronounce:
> > The simple way seems to be to malloc a large area, touch every page and
> > then look at the physical pages assigned ... they now mostly seem to be
> > descending in physical address.
> > 
> 
> OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
> 

I tried using pagemap to verify the patch but it triggered BUG_ON
checks. Perhaps I am using the interface wrong but I would still not
expect it to break in this fashion. I tried 2.6.24-rc4-mm1, 2.6.24-rc5-mm1,
2.6.24-rc5 with just the maps4 patches applied and 2.6.23 with maps4 patches
applied. Each time I get errors like this;

[   90.108315] BUG: sleeping function called from invalid context at include/asm/uaccess_32.h:457
[   90.211227] in_atomic():1, irqs_disabled():0
[   90.262251] no locks held by showcontiguous/2814.
[   90.318475] Pid: 2814, comm: showcontiguous Not tainted 2.6.24-rc5 #1
[   90.395344]  [<c010522a>] show_trace_log_lvl+0x1a/0x30
[   90.456948]  [<c0105bb2>] show_trace+0x12/0x20
[   90.510173]  [<c0105eee>] dump_stack+0x6e/0x80
[   90.563409]  [<c01205b3>] __might_sleep+0xc3/0xe0
[   90.619765]  [<c02264fd>] copy_to_user+0x3d/0x60
[   90.675153]  [<c01b3e9c>] add_to_pagemap+0x5c/0x80
[   90.732513]  [<c01b43e8>] pagemap_pte_range+0x68/0xb0
[   90.793010]  [<c0175ed2>] walk_page_range+0x112/0x210
[   90.853482]  [<c01b47c6>] pagemap_read+0x176/0x220
[   90.910863]  [<c0182dc4>] vfs_read+0x94/0x150
[   90.963058]  [<c01832fd>] sys_read+0x3d/0x70
[   91.014219]  [<c0104262>] syscall_call+0x7/0xb
[   91.067433]  =======================
[   91.110137] BUG: scheduling while atomic: showcontiguous/2814/0x00000001
[   91.190169] no locks held by showcontiguous/2814.
[   91.246293] Pid: 2814, comm: showcontiguous Not tainted 2.6.24-rc5 #1
[   91.323145]  [<c010522a>] show_trace_log_lvl+0x1a/0x30
[   91.384633]  [<c0105bb2>] show_trace+0x12/0x20
[   91.437878]  [<c0105eee>] dump_stack+0x6e/0x80
[   91.491116]  [<c0123816>] __schedule_bug+0x66/0x70
[   91.548467]  [<c033ba96>] schedule+0x556/0x7b0
[   91.601698]  [<c01042e6>] work_resched+0x5/0x21
[   91.655977]  =======================
[   91.704927] showcontiguous[2814]: segfault at b7eaa900 eip b7eaa900 esp bfa02e8c error 4
[   91.801633] BUG: scheduling while atomic: showcontiguous/2814/0x00000001
[   91.881634] no locks held by showcontiguous/2814.
[   91.937779] Pid: 2814, comm: showcontiguous Not tainted 2.6.24-rc5 #1
[   92.014606]  [<c010522a>] show_trace_log_lvl+0x1a/0x30
[   92.076123]  [<c0105bb2>] show_trace+0x12/0x20
[   92.129354]  [<c0105eee>] dump_stack+0x6e/0x80
[   92.182567]  [<c0123816>] __schedule_bug+0x66/0x70
[   92.239959]  [<c033ba96>] schedule+0x556/0x7b0
[   92.293187]  [<c01042e6>] work_resched+0x5/0x21
[   92.347452]  =======================
[   92.392697] note: showcontiguous[2814] exited with preempt_count 1
[   92.468611] BUG: scheduling while atomic: showcontiguous/2814/0x10000001
[   92.548588] no locks held by showcontiguous/2814.
[   92.604732] Pid: 2814, comm: showcontiguous Not tainted 2.6.24-rc5 #1
[   92.681665]  [<c010522a>] show_trace_log_lvl+0x1a/0x30
[   92.743180]  [<c0105bb2>] show_trace+0x12/0x20
[   92.796409]  [<c0105eee>] dump_stack+0x6e/0x80
[   92.849621]  [<c0123816>] __schedule_bug+0x66/0x70
[   92.907014]  [<c033ba96>] schedule+0x556/0x7b0
[   92.960349]  [<c0123847>] __cond_resched+0x27/0x40
[   93.017804]  [<c033be3a>] cond_resched+0x2a/0x40
[   93.073122]  [<c016e22c>] unmap_vmas+0x4ec/0x540
[   93.128418]  [<c017132f>] exit_mmap+0x6f/0xf0
[   93.180611]  [<c01254d1>] mmput+0x31/0xb0
[   93.228665]  [<c01295fd>] exit_mm+0x8d/0xf0
[   93.278788]  [<c012ac8f>] do_exit+0x15f/0x7e0
[   93.330965]  [<c012b339>] do_group_exit+0x29/0x70
[   93.387321]  [<c0133e07>] get_signal_to_deliver+0x2b7/0x490
[   93.454013]  [<c010373d>] do_notify_resume+0x7d/0x760
[   93.514476]  [<c0104315>] work_notifysig+0x13/0x1a
[   93.571869]  =======================

Just using cp to read the file is enough to cause problems but I included
a very basic program below that produces the BUG_ON checks. Is this a known
issue or am I using the interface incorrectly?

#include <stdio.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/types.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define MAPSIZE (4*1048576)
#define PM_ENTRY_BYTES sizeof(__u64)

int main(int argc, char **argv)
{
	int pagemap_fd;
	unsigned long *anonmapping;
	__u64 pagemap_entry = 0ULL;

	unsigned long vpfn, ppfn;
	size_t mmap_offset;
	int pagesize = getpagesize();

	/* Open the pagemap interface */
	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
	if (pagemap_fd == -1) {
		perror("fopen");
		exit(EXIT_FAILURE);
	}

	/* Create an anonymous mapping */
	anonmapping = mmap(NULL, MAPSIZE,
			PROT_READ|PROT_WRITE,
			MAP_PRIVATE|MAP_ANONYMOUS|MAP_POPULATE,
			-1, 0);
	if (anonmapping == MAP_FAILED) {
		perror("mmap");
		exit(1);
	}

	/* Work out the VPN the mapping is at and seek to it in pagemap */
	vpfn = ((unsigned long)anonmapping) / pagesize;
	mmap_offset = lseek(pagemap_fd, vpfn * PM_ENTRY_BYTES, SEEK_SET);
	if (mmap_offset == -1) {
		perror("fseek");
		exit(EXIT_FAILURE);
	}

	/* Read the PFN of each page in the mapping */
	for (mmap_offset = 0; mmap_offset < MAPSIZE; mmap_offset += pagesize) {
		vpfn = ((unsigned long)anonmapping + mmap_offset) / pagesize;

		if (read(pagemap_fd, &pagemap_entry, PM_ENTRY_BYTES) == 0) {
			perror("fread");
			exit(EXIT_FAILURE);
		}

		ppfn = (unsigned long)pagemap_entry;
		printf("vpfn = %8lu ppfn = %8lu\n", vpfn, ppfn);
	}

	close(pagemap_fd);
	munmap(anonmapping, MAPSIZE);
	exit(EXIT_SUCCESS);
}

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Andrew Morton]

On Sat, 15 Dec 2007 01:09:41 +0000 Mel Gorman <mel@csn.ul.ie> wrote:

> On (13/12/07 14:29), Andrew Morton didst pronounce:
> > > The simple way seems to be to malloc a large area, touch every page and
> > > then look at the physical pages assigned ... they now mostly seem to be
> > > descending in physical address.
> > > 
> > 
> > OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
> > 
> 
> I tried using pagemap to verify the patch but it triggered BUG_ON
> checks. Perhaps I am using the interface wrong but I would still not
> expect it to break in this fashion. I tried 2.6.24-rc4-mm1, 2.6.24-rc5-mm1,
> 2.6.24-rc5 with just the maps4 patches applied and 2.6.23 with maps4 patches
> applied. Each time I get errors like this;
> 
> [   90.108315] BUG: sleeping function called from invalid context at include/asm/uaccess_32.h:457
> [   90.211227] in_atomic():1, irqs_disabled():0
> [   90.262251] no locks held by showcontiguous/2814.
> [   90.318475] Pid: 2814, comm: showcontiguous Not tainted 2.6.24-rc5 #1
> [   90.395344]  [<c010522a>] show_trace_log_lvl+0x1a/0x30
> [   90.456948]  [<c0105bb2>] show_trace+0x12/0x20
> [   90.510173]  [<c0105eee>] dump_stack+0x6e/0x80
> [   90.563409]  [<c01205b3>] __might_sleep+0xc3/0xe0
> [   90.619765]  [<c02264fd>] copy_to_user+0x3d/0x60
> [   90.675153]  [<c01b3e9c>] add_to_pagemap+0x5c/0x80
> [   90.732513]  [<c01b43e8>] pagemap_pte_range+0x68/0xb0
> [   90.793010]  [<c0175ed2>] walk_page_range+0x112/0x210
> [   90.853482]  [<c01b47c6>] pagemap_read+0x176/0x220
> [   90.910863]  [<c0182dc4>] vfs_read+0x94/0x150
> [   90.963058]  [<c01832fd>] sys_read+0x3d/0x70
> [   91.014219]  [<c0104262>] syscall_call+0x7/0xb
> 
> ...
>
> Just using cp to read the file is enough to cause problems but I included
> a very basic program below that produces the BUG_ON checks. Is this a known
> issue or am I using the interface incorrectly?

I'd say you're using it correctly but you've found a hitherto unknown bug. 
On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
presently does.

Drat.

Still, that shouldn't really disrupt the testing which you're doing.  You
could disable CONFIG_HIGHPTE to shut it up.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Matt Mackall]

On Fri, Dec 14, 2007 at 06:02:06PM -0800, Andrew Morton wrote:
> On Sat, 15 Dec 2007 01:09:41 +0000 Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > On (13/12/07 14:29), Andrew Morton didst pronounce:
> > > > The simple way seems to be to malloc a large area, touch every page and
> > > > then look at the physical pages assigned ... they now mostly seem to be
> > > > descending in physical address.
> > > > 
> > > 
> > > OIC.  -mm's /proc/pid/pagemap can be used to get the pfn's...
> > > 
> > 
> > I tried using pagemap to verify the patch but it triggered BUG_ON
> > checks. Perhaps I am using the interface wrong but I would still not
> > expect it to break in this fashion. I tried 2.6.24-rc4-mm1, 2.6.24-rc5-mm1,
> > 2.6.24-rc5 with just the maps4 patches applied and 2.6.23 with maps4 patches
> > applied. Each time I get errors like this;
> > 
> > [   90.108315] BUG: sleeping function called from invalid context at include/asm/uaccess_32.h:457
> > [   90.211227] in_atomic():1, irqs_disabled():0
> > [   90.262251] no locks held by showcontiguous/2814.
> > [   90.318475] Pid: 2814, comm: showcontiguous Not tainted 2.6.24-rc5 #1
> > [   90.395344]  [<c010522a>] show_trace_log_lvl+0x1a/0x30
> > [   90.456948]  [<c0105bb2>] show_trace+0x12/0x20
> > [   90.510173]  [<c0105eee>] dump_stack+0x6e/0x80
> > [   90.563409]  [<c01205b3>] __might_sleep+0xc3/0xe0
> > [   90.619765]  [<c02264fd>] copy_to_user+0x3d/0x60
> > [   90.675153]  [<c01b3e9c>] add_to_pagemap+0x5c/0x80
> > [   90.732513]  [<c01b43e8>] pagemap_pte_range+0x68/0xb0
> > [   90.793010]  [<c0175ed2>] walk_page_range+0x112/0x210
> > [   90.853482]  [<c01b47c6>] pagemap_read+0x176/0x220
> > [   90.910863]  [<c0182dc4>] vfs_read+0x94/0x150
> > [   90.963058]  [<c01832fd>] sys_read+0x3d/0x70
> > [   91.014219]  [<c0104262>] syscall_call+0x7/0xb
> > 
> > ...
> >
> > Just using cp to read the file is enough to cause problems but I included
> > a very basic program below that produces the BUG_ON checks. Is this a known
> > issue or am I using the interface incorrectly?
> 
> I'd say you're using it correctly but you've found a hitherto unknown bug. 
> On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
> takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
> presently does.

Damn, I coulda sworn I fixed that.

-- 
Mathematics is the supreme nostalgia of our time.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mel Gorman]

> > Just using cp to read the file is enough to cause problems but I included
> > a very basic program below that produces the BUG_ON checks. Is this a known
> > issue or am I using the interface incorrectly?
> 
> I'd say you're using it correctly but you've found a hitherto unknown bug. 
> On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
> takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
> presently does.
> 
> Drat.
> 
> Still, that shouldn't really disrupt the testing which you're doing.  You
> could disable CONFIG_HIGHPTE to shut it up.
> 

Yes, that did the trick. Using pagemap, it was trivial to show that the
2.6.24-rc5-mm1 kernel was placing pages in reverse physical order like
the following output shows

b:  32763 v:   753091 p:    65559 . 65558 contig: 1
b:  32764 v:   753092 p:    65558 . 65557 contig: 1
b:  32765 v:   753093 p:    65557 . 65556 contig: 1
b:  32766 v:   753094 p:    65556 . 65555 contig: 1
b:  32767 v:   753095 p:    65555 . 65555 contig: 1

p: is the PFN of the page v: is the page offset within an anonymous
mapping and b: is the number of non-contiguous blocks in the anonymous
mapping. With the patch applied, it looks more like;

b:   1232 v:   752964 p:    58944 ................ 87328 contig: 15
b:   1233 v:   752980 p:    87328 ................ 91200 contig: 15
b:   1234 v:   752996 p:    91200 ................ 40272 contig: 15
b:   1235 v:   753012 p:    40272 ................ 85664 contig: 15
b:   1236 v:   753028 p:    85664 ................ 87312 contig: 15

so mappings are using contiguous pages again. This was the final test
program I used in case it's of any interest.

Thanks

/*
 * showcontiguous.c
 *
 * Use the /proc/pid/pagemap interface to give an indication of how contiguous
 * physical memory is in an anonymous virtual memory mapping
 */
#include <stdio.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/types.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define MAPSIZE (128*1048576)
#define PM_ENTRY_BYTES sizeof(__u64)

int main(int argc, char **argv)
{
	int pagemap_fd;
	unsigned long *anonmapping;
	__u64 pagemap_entry = 0ULL;

	unsigned long vpfn, ppfn, ppfn_last;
	int block_number = 0;
	int contig_count = 1;
	size_t mmap_offset;
	int pagesize = getpagesize();

	if (sizeof(pagemap_entry) < PM_ENTRY_BYTES) {
		printf("ERROR: Failed assumption on size of pagemap_entry\n");
		exit(EXIT_FAILURE);
	}

	/* Open the pagemap interface */
	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
	if (pagemap_fd == -1) {
		perror("fopen");
		exit(EXIT_FAILURE);
	}

	/* Create an anonymous mapping */
	anonmapping = mmap(NULL, MAPSIZE,
			PROT_READ|PROT_WRITE,
			MAP_PRIVATE|MAP_ANONYMOUS|MAP_POPULATE,
			-1, 0);
	if (anonmapping == MAP_FAILED) {
		perror("mmap");
		exit(1);
	}

	/* Work out the VPN the mapping is at and seek to it in pagemap */
	vpfn = ((unsigned long)anonmapping) / pagesize;
	mmap_offset = lseek(pagemap_fd, vpfn * PM_ENTRY_BYTES, SEEK_SET);
	if (mmap_offset == -1) {
		perror("fseek");
		exit(EXIT_FAILURE);
	}
	ppfn_last = 0;

	/* Read the PFN of each page in the mapping */
	for (mmap_offset = 0; mmap_offset < MAPSIZE; mmap_offset += pagesize) {
		vpfn = ((unsigned long)anonmapping + mmap_offset) / pagesize;

		if (read(pagemap_fd, &pagemap_entry, PM_ENTRY_BYTES) == 0) {
			perror("fread");
			exit(EXIT_FAILURE);
		}

		ppfn = (unsigned long)pagemap_entry;
		if (ppfn == ppfn_last + 1) {
			printf(".");
			contig_count++;
		} else {
			printf(" %lu contig: %d\nb: %6d v: %8lu p: %8lu .",
				ppfn, contig_count,
				block_number, vpfn, ppfn);
			contig_count = 1;
			block_number++;
		}
		ppfn_last = ppfn;
	}
	printf(" %lu config: %d\n", ppfn, contig_count);

	close(pagemap_fd);
	munmap(anonmapping, MAPSIZE);
	exit(EXIT_SUCCESS);
}
-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Randy Dunlap]

On Sun, 16 Dec 2007 21:55:20 +0000 Mel Gorman wrote:

> > > Just using cp to read the file is enough to cause problems but I included
> > > a very basic program below that produces the BUG_ON checks. Is this a known
> > > issue or am I using the interface incorrectly?
> > 
> > I'd say you're using it correctly but you've found a hitherto unknown bug. 
> > On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
> > takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
> > presently does.
> > 
> > Drat.
> > 
> > Still, that shouldn't really disrupt the testing which you're doing.  You
> > could disable CONFIG_HIGHPTE to shut it up.
> > 
> 
> Yes, that did the trick. Using pagemap, it was trivial to show that the
> 2.6.24-rc5-mm1 kernel was placing pages in reverse physical order like
> the following output shows
> 
> b:  32763 v:   753091 p:    65559 . 65558 contig: 1
> b:  32764 v:   753092 p:    65558 . 65557 contig: 1
> b:  32765 v:   753093 p:    65557 . 65556 contig: 1
> b:  32766 v:   753094 p:    65556 . 65555 contig: 1
> b:  32767 v:   753095 p:    65555 . 65555 contig: 1
> 
> p: is the PFN of the page v: is the page offset within an anonymous
> mapping and b: is the number of non-contiguous blocks in the anonymous
> mapping. With the patch applied, it looks more like;
> 
> b:   1232 v:   752964 p:    58944 ................ 87328 contig: 15
> b:   1233 v:   752980 p:    87328 ................ 91200 contig: 15
> b:   1234 v:   752996 p:    91200 ................ 40272 contig: 15
> b:   1235 v:   753012 p:    40272 ................ 85664 contig: 15
> b:   1236 v:   753028 p:    85664 ................ 87312 contig: 15
> 
> so mappings are using contiguous pages again. This was the final test
> program I used in case it's of any interest.
> 
> Thanks
> 
> /*
>  * showcontiguous.c
>  *
>  * Use the /proc/pid/pagemap interface to give an indication of how contiguous
>  * physical memory is in an anonymous virtual memory mapping
>  */

Matt,
Did you ever make your python pagemap scripts available?
If not, would you?

---
~Randy

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Matt Mackall]

On Mon, Dec 17, 2007 at 11:24:57AM -0800, Randy Dunlap wrote:
> On Sun, 16 Dec 2007 21:55:20 +0000 Mel Gorman wrote:
> 
> > > > Just using cp to read the file is enough to cause problems but I included
> > > > a very basic program below that produces the BUG_ON checks. Is this a known
> > > > issue or am I using the interface incorrectly?
> > > 
> > > I'd say you're using it correctly but you've found a hitherto unknown bug. 
> > > On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
> > > takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
> > > presently does.
> > > 
> > > Drat.
> > > 
> > > Still, that shouldn't really disrupt the testing which you're doing.  You
> > > could disable CONFIG_HIGHPTE to shut it up.
> > > 
> > 
> > Yes, that did the trick. Using pagemap, it was trivial to show that the
> > 2.6.24-rc5-mm1 kernel was placing pages in reverse physical order like
> > the following output shows
> > 
> > b:  32763 v:   753091 p:    65559 . 65558 contig: 1
> > b:  32764 v:   753092 p:    65558 . 65557 contig: 1
> > b:  32765 v:   753093 p:    65557 . 65556 contig: 1
> > b:  32766 v:   753094 p:    65556 . 65555 contig: 1
> > b:  32767 v:   753095 p:    65555 . 65555 contig: 1
> > 
> > p: is the PFN of the page v: is the page offset within an anonymous
> > mapping and b: is the number of non-contiguous blocks in the anonymous
> > mapping. With the patch applied, it looks more like;
> > 
> > b:   1232 v:   752964 p:    58944 ................ 87328 contig: 15
> > b:   1233 v:   752980 p:    87328 ................ 91200 contig: 15
> > b:   1234 v:   752996 p:    91200 ................ 40272 contig: 15
> > b:   1235 v:   753012 p:    40272 ................ 85664 contig: 15
> > b:   1236 v:   753028 p:    85664 ................ 87312 contig: 15
> > 
> > so mappings are using contiguous pages again. This was the final test
> > program I used in case it's of any interest.
> > 
> > Thanks
> > 
> > /*
> >  * showcontiguous.c
> >  *
> >  * Use the /proc/pid/pagemap interface to give an indication of how contiguous
> >  * physical memory is in an anonymous virtual memory mapping
> >  */
> 
> Matt,
> Did you ever make your python pagemap scripts available?
> If not, would you?

There's a collection of them at http://selenic.com/repo/pagemap.
They're largely proof of concept, and I'm not sure I finished adapting
them all to the final 64-bit interface.

As it happens, the above regression I actually spotted immediately by
doing a simple hexdump on my very first test of the interface - lots
of pfns counting backwards. Mentioned it a few times to various people
in the cc: list and on lkml but never got around to tracking it down
myself..

-- 
Mathematics is the supreme nostalgia of our time.

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Andrew Morton wrote:
> On Thu, 13 Dec 2007 21:09:59 +0100
> Jens Axboe <jens.axboe@oracle.com> wrote:
> 
> >
> > OK, it's a vm issue,
> 
> cc linux-mm and probable culprit.
> 
> >  I have tens of thousand "backward" pages after a
> > boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> > reverse. So it looks like that bug got reintroduced.
> 
> Bill Irwin fixed this a couple of years back: changed the page allocator so
> that it mostly hands out pages in ascending physical-address order.
> 
> I guess we broke that, quite possibly in Mel's page allocator rework.
> 
> It would help if you could provide us with a simple recipe for
> demonstrating this problem, please.

Basically anything involving IO :-). A boot here showed a handful of
good merges, and probably in the order of 100,000 descending
allocations. A kernel make is a fine test as well.

Something like the below should work fine - if you see oodles of these
basicaly doing any type of IO, then you are screwed.

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e30b1a4..8ce3fcc 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1349,6 +1349,10 @@ new_segment:
 				sg = sg_next(sg);
 			}
 
+			if (bvprv) {
+				if (page_address(bvec->bv_page) + PAGE_SIZE == page_address(bvprv->bv_page) && printk_ratelimit())
+					printk("page alloc order backwards\n");
+			}
 			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
 			nsegs++;
 		}

-- 
Jens Axboe

VM allocates pages in reverse order again

[Matthew Wilcox]

On Thu, Dec 13, 2007 at 09:09:59PM +0100, Jens Axboe wrote:
> > >diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> > >index e30b1a4..1e34b6f 100644
> > >--- a/block/ll_rw_blk.c
> > >+++ b/block/ll_rw_blk.c
> > >@@ -1349,6 +1351,8 @@ new_segment:
> > > 				sg = sg_next(sg);
> > > 			}
> > > 
> > >+			if (bvprv && (page_address(bvprv->bv_page) + 
> > >bvprv->bv_len == page_address(bvec->bv_page)))
> > >+				printk("missed merge\n");
> > > 			sg_set_page(sg, bvec->bv_page, nbytes, 
> > > 			bvec->bv_offset);
> > > 			nsegs++;
> > > 		}
> > >
> > ..
> > 
> > Yeah, the first part is similar to my own hack.
> > 
> > For testing, try "dd if=/dev/sda of=/dev/null bs=4096k".
> > That *really* should end up using contiguous pages on most systems.
> > 
> > I figured out the git thing, and am now building some in-between kernels to 
> > try.
> 
> OK, it's a vm issue, I have tens of thousand "backward" pages after a
> boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> reverse. So it looks like that bug got reintroduced.

Perhaps we should ask the -mm folks if they happen to have an idea what
caused it ...

Background: we're seeing pages allocated in reverse order after boot.
This causes IO performance problems on machines without IOMMUs as we
can't merge pages when they're allocated in the wrong order.  This is
something that went wrong between 2.6.23 and 2.6.24-rc5.

Bill Irwin had a patch that fixed this; it was merged months ago, but
the effects of it seem to have been undone.

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe]

On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Mark Lord wrote:
> >>Matthew Wilcox wrote:
> >>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB 
> >>>>for libata,
> >>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>Just a suspicion ... could this be slab vs slub?  ie check your configs
> >>>are the same / similar between the two kernels.
> >>..
> >>
> >>Mmmm.. a good thought, that one.
> >>But I just rechecked, and both have CONFIG_SLAB=y
> >>
> >>My guess is that something got changed around when Jens
> >>reworked the block layer for 2.6.24.
> >>I'm going to dig around in there now.
> >
> >I didn't rework the block layer for 2.6.24 :-). The core block layer
> >changes since 2.6.23 are:
> >
> >- Support for empty barriers. Not a likely candidate.
> >- Shared tag queue fixes. Totally unlikely.
> >- sg chaining support. Not likely.
> >- The bio changes from Neil. Of the bunch, the most likely suspects in
> >  this area, since it changes some of the code involved with merges and
> >  blk_rq_map_sg().
> >- Lots of simple stuff, again very unlikely.
> >
> >Anyway, it sounds odd for this to be a block layer problem if you do see
> >occasional segments being merged. So it sounds more like the input data
> >having changed.
> >
> >Why not just bisect it?
> ..
> 
> Because the early 2.6.24 series failed to boot on this machine
> due to bugs in the block layer -- so the code that caused this regression
> is probably in the stuff from before the kernels became usable here.

That would be the sg chain stuff, I don't think there's any correlation
between the two "bugs" (ie I don't get how you jump to the conclusion
that this regression is from stuff before that). Just go back as early
as you can, you could even just start with a -rc bisect.

-- 
Jens Axboe

Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord]

Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
>> Matthew Wilcox wrote:
>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB for 
>>>> libata,
>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>> Just a suspicion ... could this be slab vs slub?  ie check your configs
>>> are the same / similar between the two kernels.
>> ..
>>
>> Mmmm.. a good thought, that one.
>> But I just rechecked, and both have CONFIG_SLAB=y
>>
>> My guess is that something got changed around when Jens
>> reworked the block layer for 2.6.24.
>> I'm going to dig around in there now.
> 
> I didn't rework the block layer for 2.6.24 :-). The core block layer
> changes since 2.6.23 are:
> 
> - Support for empty barriers. Not a likely candidate.
> - Shared tag queue fixes. Totally unlikely.
> - sg chaining support. Not likely.
> - The bio changes from Neil. Of the bunch, the most likely suspects in
>   this area, since it changes some of the code involved with merges and
>   blk_rq_map_sg().
> - Lots of simple stuff, again very unlikely.
> 
> Anyway, it sounds odd for this to be a block layer problem if you do see
> occasional segments being merged. So it sounds more like the input data
> having changed.
> 
> Why not just bisect it?
..

CC'ing Neil Brown.