system call for process information?

system call for process information?

[Guennadi Liakhovetski]

Hello

I asked this question on kernel-newbies - no reply, hope to be luckier
here:-)

I need to collect some info on processes. One way is to read /proc
tree. But isn't there a system call (ioctl) for this? And what are those
task[], task_struct, etc. about?

Thanks
Guennadi
___

Dr. Guennadi V. Liakhovetski
Department of Applied Mathematics
University of Sheffield, U.K.
email: G.Liakhovetski@sheffield.ac.uk


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Re: system call for process information?

[Alexander Viro]

On Mon, 12 Mar 2001, Guennadi Liakhovetski wrote:

> Hello
> 
> I asked this question on kernel-newbies - no reply, hope to be luckier
> here:-)
> 
> I need to collect some info on processes. One way is to read /proc
> tree. But isn't there a system call (ioctl) for this? And what are those

Occam's Razor.  Why invent new syscall when read() works?

> task[], task_struct, etc. about?

What branch? (2.0, 2.2, 2.4?)
							Cheers,
								Al

Re: system call for process information?

[Guennadi Liakhovetski]

On Mon, 12 Mar 2001, Alexander Viro wrote:

> On Mon, 12 Mar 2001, Guennadi Liakhovetski wrote:
> 
> > I need to collect some info on processes. One way is to read /proc
> > tree. But isn't there a system call (ioctl) for this? And what are those
> 
> Occam's Razor.  Why invent new syscall when read() works?

CPU utilisation. Each new application has to calculate it (ps, top, qps,
kps, various sysmons, procmons, etc.). Wouldn't it be worth it having a
syscall for that? Wouldn't it be more optimal?

> > task[], task_struct, etc. about?
> 
> What branch? (2.0, 2.2, 2.4?)

Well, what I mean was - don't these structures contain the information I
am looking for? Let's start from the end - 2.4, then what's the difference
with 2.2 and finally 2.0?

Thanks
Guennadi
___

Dr. Guennadi V. Liakhovetski
Department of Applied Mathematics
University of Sheffield, U.K.
email: G.Liakhovetski@sheffield.ac.uk

Re: system call for process information?

[Nathan Paul Simons]

On Mon, Mar 12, 2001 at 09:21:37PM +0000, Guennadi Liakhovetski wrote:
> CPU utilisation. Each new application has to calculate it (ps, top, qps,
> kps, various sysmons, procmons, etc.). Wouldn't it be worth it having a
> syscall for that? Wouldn't it be more optimal?

	No, it wouldn't be worth it because you're talking about 
sacrificing simplicity and kernel speed in favor of functionality.
This has been know to lead to "bloat-ware".  Every new syscall you
add takes just a little bit more time and space in the kernel, and
when only a small number of programs will be using it, it's really 
not worth it.  This time and space may not be large, but once you
get _your_ syscall added, why can't everyone else get theirs added 
as well?  And so, after making about a thousand specialized syscalls
standard in the kernel, you end up with IRIX (from what I've heard).
	Don't even get me started about opening security holes, and
increasing code complexity.  Please do a search for every other
syscall that has ever been proposed on this list, read them all 
and the arguments for them, then think long and hard about why yours
should be accepted.  Because I'm sure that I'm not the only person
who's going to want a good explanation as to why this syscall is
essential.

ps - CPU time is cheap, that's why they don't charge for it anymore.
Programmer time is _not_.

Re: system call for process information?

[Alexander Viro]

On Mon, 12 Mar 2001, Nathan Paul Simons wrote:

> On Mon, Mar 12, 2001 at 09:21:37PM +0000, Guennadi Liakhovetski wrote:
> > CPU utilisation. Each new application has to calculate it (ps, top, qps,
> > kps, various sysmons, procmons, etc.). Wouldn't it be worth it having a
> > syscall for that? Wouldn't it be more optimal?

The first rule of optimization: don't. I.e. optimizing something that
is not a bottleneck is pointless.


> 	No, it wouldn't be worth it because you're talking about 
> sacrificing simplicity and kernel speed in favor of functionality.

Or, in that case, in favour of nothing. It doesn't add any functionality.

> This has been know to lead to "bloat-ware".  Every new syscall you
> add takes just a little bit more time and space in the kernel, and
> when only a small number of programs will be using it, it's really 
> not worth it.  This time and space may not be large, but once you
> get _your_ syscall added, why can't everyone else get theirs added 
> as well?  And so, after making about a thousand specialized syscalls
> standard in the kernel, you end up with IRIX (from what I've heard).

	In that case there is much simpler argument.

If your program checks the system load so often that converting results
from ASCII to integers takes noticable time - all you are measuring
is the load created by that program. In other words, any program that
would get any speedup from such syscall is absolutely worthless, since
the load created by measurement will drown the load you are trying
to measure.

	End of story. It's not only unnecessary and tasteless, it's
useless. 
								Cheers,
									Al

Re: system call for process information?

[Guennadi Liakhovetski]

Hi Alexander, Nathan and all!

Thanks for your great answers! First of all - I was not REALLY proposing
to include this system call in the kernel - I just wanted to hear some pro
and contra - so, thanks again for your explanations! I started yesterday
sketching the required functions, will have to retreat to reading top & ps
sources, btw, apart from these 2 obvious sources, what else would you
suggest to look through for a good implementation of CPU-utilization
calculator as well as other process (multithreaded, SMP,...) statistics?
Portable (POSIX), maybe some documentation, not just sources?

Thanks
Guennadi

On Mon, 12 Mar 2001, Alexander Viro wrote:

> On Mon, 12 Mar 2001, Nathan Paul Simons wrote:
> 
> > On Mon, Mar 12, 2001 at 09:21:37PM +0000, Guennadi Liakhovetski wrote:
> > > CPU utilisation. Each new application has to calculate it (ps, top, qps,
> > > kps, various sysmons, procmons, etc.). Wouldn't it be worth it having a
> > > syscall for that? Wouldn't it be more optimal?
> 
> The first rule of optimization: don't. I.e. optimizing something that
> is not a bottleneck is pointless.
> 
> > 	No, it wouldn't be worth it because you're talking about 
> > sacrificing simplicity and kernel speed in favor of functionality.
> 
> Or, in that case, in favour of nothing. It doesn't add any functionality.
> 
> > This has been know to lead to "bloat-ware".  Every new syscall you
> > add takes just a little bit more time and space in the kernel, and
> > when only a small number of programs will be using it, it's really 
> > not worth it.  This time and space may not be large, but once you
> > get _your_ syscall added, why can't everyone else get theirs added 
> > as well?  And so, after making about a thousand specialized syscalls
> > standard in the kernel, you end up with IRIX (from what I've heard).
> 
> 	In that case there is much simpler argument.
> 
> If your program checks the system load so often that converting results
> from ASCII to integers takes noticable time - all you are measuring
> is the load created by that program. In other words, any program that
> would get any speedup from such syscall is absolutely worthless, since
> the load created by measurement will drown the load you are trying
> to measure.
> 
> 	End of story. It's not only unnecessary and tasteless, it's
> useless. 
> 								Cheers,
> 									Al
> 
> 

___

Dr. Guennadi V. Liakhovetski
Department of Applied Mathematics
University of Sheffield, U.K.
email: G.Liakhovetski@sheffield.ac.uk

Re: system call for process information?

[Albert D. Cahalan]

Nathan Paul Simons writes:
> On Mon, Mar 12, 2001 at 09:21:37PM +0000, Guennadi Liakhovetski wrote:

>> CPU utilisation. Each new application has to calculate it (ps, top, qps,
>> kps, various sysmons, procmons, etc.). Wouldn't it be worth it having a
>> syscall for that? Wouldn't it be more optimal?
>
> 	No, it wouldn't be worth it because you're talking about 
> sacrificing simplicity and kernel speed in favor of functionality.
> This has been know to lead to "bloat-ware".  Every new syscall you

Bloat removal: being able to run without /proc mounted.

We don't have "kernel speed". We have kernel-mode screwing around
with text formatting.

> add takes just a little bit more time and space in the kernel, and
> when only a small number of programs will be using it, it's really 
> not worth it.  This time and space may not be large, but once you
> get _your_ syscall added, why can't everyone else get theirs added 
> as well?  And so, after making about a thousand specialized syscalls
> standard in the kernel, you end up with IRIX (from what I've heard).

This isn't just for him. Many people have wanted it.

> 	Don't even get me started about opening security holes, and
> increasing code complexity.  Please do a search for every other

I'll get you started. Compare:

1. variable-length ASCII strings with undefined ad-hoc syntax
2. array of fixed-size (64-bit) values

> ps - CPU time is cheap, that's why they don't charge for it anymore.
> Programmer time is _not_.

Parsing costs programmer time.

Re: system call for process information?

[Nathan Paul Simons]

On Tue, Mar 13, 2001 at 04:05:13PM -0500, Albert D. Cahalan wrote:
> Bloat removal: being able to run without /proc mounted.
> 
> We don't have "kernel speed". We have kernel-mode screwing around
> with text formatting.

	Or calculating things that really should be taken care of in
user space, such as CPU utilization.

> This isn't just for him. Many people have wanted it.

	Yes, but how many people would actually *use* it?  How many
programs out of the thousands out there would benefit from this?
If it's more than 50 widely used packages, I'd be more than happy
to see something that speeds them all up added to the kernel.

> 1. variable-length ASCII strings with undefined ad-hoc syntax

	Use enumerated string functions, always.

> 2. array of fixed-size (64-bit) values

	It's an array?  That can still be overflowed by sloppy 
programming.  When it comes right down to it, I'd rather have 
something that could potentially die badly be run on the user
side, rather than the kernel side.

> Parsing costs programmer time.

	But it's fairly easy to do in any number of programming
languages besides C which can't be easily used in the kernel.
Not to mention parsing libraries for C that fit much better on
the user side because they would make the kernel huge and slow
if compiled into it.
	Last but not least, I don't want to waste time in kernel
scanning through a list of syscalls a mile long, half of them
I don't ever use.  Or having a kernel that's so big that you
can't fit it on embedded systems anymore.  And once you start
adding every "nifty" syscall that comes along, that's what
will happen.  So again, I say give us all a really good reason
for this syscall, or just hack it into your own kernels and 
let us have our speedy, small vanilla kernels.

Re: system call for process information?

[Albert D. Cahalan]

Nathan Paul Simons writes:
> On Tue, Mar 13, 2001 at 04:05:13PM -0500, Albert D. Cahalan wrote:

>> Bloat removal: being able to run without /proc mounted.
>>
>> We don't have "kernel speed". We have kernel-mode screwing around
>> with text formatting.
>
> 	Or calculating things that really should be taken care of in
> user space, such as CPU utilization.

That can not be done reliably in user space. I know this; the "top"
program used to try.

>> This isn't just for him. Many people have wanted it.
>
> 	Yes, but how many people would actually *use* it?  How many
> programs out of the thousands out there would benefit from this?
> If it's more than 50 widely used packages, I'd be more than happy
> to see something that speeds them all up added to the kernel.

Oh please. How many programs use the mount() system call? One?
Most system calls are rarely used. This is OK.

>> 1. variable-length ASCII strings with undefined ad-hoc syntax
>
> 	Use enumerated string functions, always.
>
>> 2. array of fixed-size (64-bit) values
>
> 	It's an array?  That can still be overflowed by sloppy
> programming.

No it can't. You fill it like this:

tmp[0] = p->pid;
tmp[1] = p->uid;
/* ... */

Throw in some pretty symbolic names if you like. It's effectively
a struct, but a real struct would tempt people to use non-64-bit
values. Using an array enforces uniform 64-bit usage.

Good design involves NOT tempting people to write irregular hacks.

>  When it comes right down to it, I'd rather have
> something that could potentially die badly be run on the user
> side, rather than the kernel side.

Good. Thus you'd like the new system call in place of our
current pile of crud. Unfortunately the crud will need to
remain for at least a decade of transition time.

>> Parsing costs programmer time.
>
> 	But it's fairly easy to do in any number of programming
> languages besides C which can't be easily used in the kernel.
> Not to mention parsing libraries for C that fit much better on
> the user side because they would make the kernel huge and slow
> if compiled into it.

Huh? The kernel need not parse its own ASCII output. The kernel
natively maintains information in a binary format. The proposed
system call would not parse /proc output!!!

> 	Last but not least, I don't want to waste time in kernel
> scanning through a list of syscalls a mile long, half of them
> I don't ever use.

Well, tough luck. Learn to use an editor with search ability.
Even "less" and Netscape can search.

>  Or having a kernel that's so big that you
> can't fit it on embedded systems anymore.

The proposed system call was implemented for an embedded system.
This allowed operation without the /proc filesystem, which is
some serious bloat.

> And once you start
> adding every "nifty" syscall that comes along, that's what
> will happen.  So again, I say give us all a really good reason
> for this syscall, or just hack it into your own kernels and
> let us have our speedy, small vanilla kernels.

If you think /proc is speedy and small...

Re: system call for process information?

[Rik van Riel]

On Tue, 13 Mar 2001, Albert D. Cahalan wrote:

> Bloat removal: being able to run without /proc mounted.
> 
> We don't have "kernel speed". We have kernel-mode screwing around
> with text formatting.

Sounds like you might want to maintain an external patch
for the embedded folks...

regards,

Rik
--
Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
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Re: system call for process information?

[Martin Dalecki]

Rik van Riel wrote:
> 
> On Tue, 13 Mar 2001, Albert D. Cahalan wrote:
> 
> > Bloat removal: being able to run without /proc mounted.
> >
> > We don't have "kernel speed". We have kernel-mode screwing around
> > with text formatting.
> 
> Sounds like you might want to maintain an external patch
> for the embedded folks...

Not the embedded folks!!! The server folks laugh histerically
all times they go via ssh to a trashing busy box to see what's wrong
and then they see top or ps auxe under linux never finishing they job:


> 
> regards,
> 
> Rik
> --
> Virtual memory is like a game you can't win;
> However, without VM there's truly nothing to lose...
> 
>                 http://www.surriel.com/
> http://www.conectiva.com/       http://distro.conectiva.com.br/
> 
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Re: system call for process information?

[john slee]

On Tue, Mar 13, 2001 at 07:52:41PM -0300, Rik van Riel wrote:
> On Tue, 13 Mar 2001, Albert D. Cahalan wrote:
> 
> > Bloat removal: being able to run without /proc mounted.
> >
> > We don't have "kernel speed". We have kernel-mode screwing around
> > with text formatting.
> 
> Sounds like you might want to maintain an external patch
> for the embedded folks...

or perhaps a patch to remove the non-procfs stuff from proc - leaving
just /proc/[0-9]+ and /proc/self...

that way top/ps/ still mostly work without patches and you dont have all
the other stuff that you don't need (perhaps make a separate kernfs?).

i *am* aware of the previous flamewars over this :-) but it does appear
to me a more generally useful compromise in the anti-bloat case.

j. (who likes proc as it is now)

Re: system call for process information?

[Rik van Riel]

On Wed, 14 Mar 2001, Martin Dalecki wrote:

> Not the embedded folks!!! The server folks laugh histerically all
> times they go via ssh to a trashing busy box to see what's wrong and
> then they see top or ps auxe under linux never finishing they job:

That's a separate issue.

I guess the pagefault path should have _2_ locks.

One mmap_sem protecting read-only access to the address space
and another one for write access to the adress space (to stop
races with swapout, other page faults, ...).

At the point where the pagefault sleeps on IO, it could release
the read-only lock, so vmstat, top, etc can get the statistics
they need. Only during the time the pagefaulting code is actually
messing with the address space could it block read access (to
prevent others from seeing an inconsistent state).

regards,

Rik
--
Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
http://www.conectiva.com/	http://distro.conectiva.com.br/

Re: system call for process information?

[george anzinger]

Rik van Riel wrote:
> 
> On Wed, 14 Mar 2001, Martin Dalecki wrote:
> 
> > Not the embedded folks!!! The server folks laugh histerically all
> > times they go via ssh to a trashing busy box to see what's wrong and
> > then they see top or ps auxe under linux never finishing they job:
> 
> That's a separate issue.
> 
> I guess the pagefault path should have _2_ locks.
> 
> One mmap_sem protecting read-only access to the address space
> and another one for write access to the adress space (to stop
> races with swapout, other page faults, ...).
> 
> At the point where the pagefault sleeps on IO, it could release
> the read-only lock, so vmstat, top, etc can get the statistics
> they need. Only during the time the pagefaulting code is actually
> messing with the address space could it block read access (to
> prevent others from seeing an inconsistent state).
> 
Is it REALLY necessary to prevent them from seeing an inconsistent
state?  Seems to me that in the total picture (i.e. system wide) they
will never see a consistent state, so why be concerned with a small
corner of the system.  Let them figure it out, possibly by consistency
checks, if they care.  It just seems unhealthy to demand consistency at
the cost of delays that will only make other data even more
inconsistent. And if the delay is _forever_ from a tool that may be used
to diagnose system problems...  I would rather a tool that repeatedly
showed the same inconsistent state than one that hangs because it can
not get a consistent one.

George

Re: system call for process information?

[george anzinger]

Rik van Riel wrote:
> 
> On Wed, 14 Mar 2001, george anzinger wrote:
> 
> > Is it REALLY necessary to prevent them from seeing an
> > inconsistent state?  Seems to me that in the total picture (i.e.
> > system wide) they will never see a consistent state, so why be
> > concerned with a small corner of the system.
> 
> You're right. All we need to make sure of is that the address
> space we want to print info about doesn't go away while we're
> reading the stats ...
> 
> (I think ... but we'll need to look at the procfs code in more
> detail)
> 
For what its worth:
On the last system I worked on we had a status program that maintained a
screen with interesting things such as context switches per sec, disc
i/o/sec, lan traffic/sec, ready queue length, next task (printed as
current task) and... well a whole 26X80 screen full of stuff.  The
program gathered all the data by reading system tables as quickly as
possible and THEN did the formatting/ screen update.  Having to deal
with pre formatted data would have a.) widened the capture window and
b.) been a real drag to reformat and move to the right screen location. 
We allowed programs that had the savvy to have read only access to the
kernel area to make this as fast as possible.

George

Re: system call for process information?

[Rik van Riel]

On Wed, 14 Mar 2001, george anzinger wrote:

> Is it REALLY necessary to prevent them from seeing an
> inconsistent state?  Seems to me that in the total picture (i.e.
> system wide) they will never see a consistent state, so why be
> concerned with a small corner of the system.

You're right. All we need to make sure of is that the address
space we want to print info about doesn't go away while we're
reading the stats ...

(I think ... but we'll need to look at the procfs code in more
detail)

regards,

Rik
--
Linux MM bugzilla: http://linux-mm.org/bugzilla.shtml

Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
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Re: system call for process information?

[Szabolcs Szakacsits]

On Mon, 12 Mar 2001, Alexander Viro wrote:
> On Mon, 12 Mar 2001, Guennadi Liakhovetski wrote:
> > I need to collect some info on processes. One way is to read /proc
> > tree. But isn't there a system call (ioctl) for this? And what are those
> Occam's Razor.  Why invent new syscall when read() works?

read() doesn't really work for this purpose, it blocks way too many
times to be very annoying. When finally data arrives it's useless.

	Szaka

Re: system call for process information?

[Alexander Viro]

On Wed, 14 Mar 2001, Szabolcs Szakacsits wrote:

> 
> On Mon, 12 Mar 2001, Alexander Viro wrote:
> > On Mon, 12 Mar 2001, Guennadi Liakhovetski wrote:
> > > I need to collect some info on processes. One way is to read /proc
> > > tree. But isn't there a system call (ioctl) for this? And what are those
> > Occam's Razor.  Why invent new syscall when read() works?
> 
> read() doesn't really work for this purpose, it blocks way too many
> times to be very annoying. When finally data arrives it's useless.

Huh? Take code of your non-blocking syscall. Make it ->read() for
relevant file on /proc or wherever else you want it. See read() not
blocking...

Whether code blocks or not depends on the code, not on the calling
conventions. And definitely not on ASCII vs. binary - conversion
between these formats _is_ doable without blocking operations.

Re: system call for process information?

[Alexander Viro]

On Wed, 14 Mar 2001, Szabolcs Szakacsits wrote:

> 
> On Wed, 14 Mar 2001, Alexander Viro wrote:
> > On Wed, 14 Mar 2001, Szabolcs Szakacsits wrote:
> > > read() doesn't really work for this purpose, it blocks way too many
> > > times to be very annoying. When finally data arrives it's useless.
> > Huh? Take code of your non-blocking syscall. Make it ->read() for
> > relevant file on /proc or wherever else you want it. See read() not
> > blocking...
> 
> Sorry I should have quoted "blocks". Problem isn't with blocking but
> *no* data, no information. In the end you can conclude you know
> *nothing* what happend in the last t time interval - this can be second,
> minutes even with an RT, mlocked, etc process when the load is around 0.

And how will a new syscall avoid the same problems you have with
read()? Again, they can share the payload code - it's a matter
of calling conventions and layout of the output. _That_ part doesn't
take long. If reading is too slow - too bad, changing the syscall
number won't help.

Re: system call for process information?

[Szabolcs Szakacsits]

On Wed, 14 Mar 2001, Alexander Viro wrote:
> On Wed, 14 Mar 2001, Szabolcs Szakacsits wrote:
> > read() doesn't really work for this purpose, it blocks way too many
> > times to be very annoying. When finally data arrives it's useless.
> Huh? Take code of your non-blocking syscall. Make it ->read() for
> relevant file on /proc or wherever else you want it. See read() not
> blocking...

Sorry I should have quoted "blocks". Problem isn't with blocking but
*no* data, no information. In the end you can conclude you know
*nothing* what happend in the last t time interval - this can be second,
minutes even with an RT, mlocked, etc process when the load is around 0.

	Szaka

changing mm->mmap_sem (was: Re: system call for process information?)

[Rik van Riel]

On Wed, 14 Mar 2001, Rik van Riel wrote:
> On Wed, 14 Mar 2001, george anzinger wrote:
> 
> > Is it REALLY necessary to prevent them from seeing an
> > inconsistent state?  Seems to me that in the total picture (i.e.
> > system wide) they will never see a consistent state, so why be
> > concerned with a small corner of the system.
> 
> You're right.

Mmmm, I've looked at the code today and it turned out that
we're NOT right ;)

The mmap_sem is used in procfs to prevent the list of VMAs
from changing. In the page fault code it seems to be used
to prevent other page faults to happen at the same time with
the current page fault (and to prevent VMAs from changing
while a page fault is underway).

Maybe we should change the mmap_sem into a R/W semaphore ?

Since page faults seem to be the "common cause" of blocking
procfs access *and* since both page faults and procfs only
need to prevent the VMA list from changing, a read lock would
help here.

Write locks would be used in the code where we actually want
to change the VMA list and page faults would use an extra lock
to protect against each other (possibly a per-pagetable lock so
multithreaded apps can pagefault in different memory regions at
the same time ???).

regards,

Rik
--
Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
http://www.conectiva.com/	http://distro.conectiva.com.br/

Re: changing mm->mmap_sem (was: Re: system call for process information?)

[Stephen C. Tweedie]

Hi,

On Thu, Mar 15, 2001 at 09:24:59AM -0300, Rik van Riel wrote:
> On Wed, 14 Mar 2001, Rik van Riel wrote:

> The mmap_sem is used in procfs to prevent the list of VMAs
> from changing. In the page fault code it seems to be used
> to prevent other page faults to happen at the same time with
> the current page fault (and to prevent VMAs from changing
> while a page fault is underway).

The page table spinlock should be quite sufficient to let us avoid
races in the page fault code.  We've had to deal with this before
there was ever a mmap_sem anyway: in ancient times, every page fault
had to do things like check to see if the pte had changed after IO was
complete and once the BKL had been retaken.  We can do the same with
the page fault spinlock without much pain.

> Maybe we should change the mmap_sem into a R/W semaphore ?

Definitely.

> Write locks would be used in the code where we actually want
> to change the VMA list and page faults would use an extra lock
> to protect against each other (possibly a per-pagetable lock

Why do we need another lock?  The critical section where we do the
final update on the pte _already_ takes the page table spinlock to
avoid races against the swapper.

Cheers,
 Stephen

Re: changing mm->mmap_sem (was: Re: system call for process information?)

[Rik van Riel]

On Fri, 16 Mar 2001, Stephen C. Tweedie wrote:
> On Thu, Mar 15, 2001 at 09:24:59AM -0300, Rik van Riel wrote:
> > On Wed, 14 Mar 2001, Rik van Riel wrote:
> 
> > The mmap_sem is used in procfs to prevent the list of VMAs
> > from changing. In the page fault code it seems to be used
> > to prevent other page faults to happen at the same time with
> > the current page fault (and to prevent VMAs from changing
> > while a page fault is underway).
> 
> The page table spinlock should be quite sufficient to let us avoid
> races in the page fault code.

> > Write locks would be used in the code where we actually want
> > to change the VMA list and page faults would use an extra lock
> > to protect against each other (possibly a per-pagetable lock
> 
> Why do we need another lock?  The critical section where we do the
> final update on the pte _already_ takes the page table spinlock to
> avoid races against the swapper.

The problem is that mmap_sem seems to be protecting the list
of VMAs, so taking _only_ the page_table_lock could let a VMA
change under us while a page fault is underway ...

Then again, I guess just making mmap_sem a R/W lock should fix
our problems ... and maybe even make it possible (in 2.5?) to
let multithreaded programs have pagefaults at the same time,
instead of having all threads queue up behind mmap_sem.

regards,

Rik
--
Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
http://www.conectiva.com/	http://distro.conectiva.com.br/

Re: changing mm->mmap_sem (was: Re: system call for process information?)

[Stephen C. Tweedie]

Hi,

On Fri, Mar 16, 2001 at 08:50:25AM -0300, Rik van Riel wrote:
> On Fri, 16 Mar 2001, Stephen C. Tweedie wrote:
> 
> > > Write locks would be used in the code where we actually want
> > > to change the VMA list and page faults would use an extra lock
> > > to protect against each other (possibly a per-pagetable lock
> > 
> > Why do we need another lock?  The critical section where we do the
> > final update on the pte _already_ takes the page table spinlock to
> > avoid races against the swapper.
> 
> The problem is that mmap_sem seems to be protecting the list
> of VMAs, so taking _only_ the page_table_lock could let a VMA
> change under us while a page fault is underway ...

Right, I'm not suggesting removing that: making the mmap_sem
read/write is fine, but yes, we still need that semaphore.  But as for
the "page faults would use an extra lock to protect against each
other" bit --- we already have another lock, the page table lock,
which can be used in this way, so ANOTHER lock should be unnecessary.

--Stephen

Re: changing mm->mmap_sem (was: Re: system call for process information?)

[Rik van Riel]

On Fri, 16 Mar 2001, Stephen C. Tweedie wrote:

> Right, I'm not suggesting removing that: making the mmap_sem
> read/write is fine, but yes, we still need that semaphore.

Initial patch (against 2.4.2-ac20) is available at
http://www.surriel.com/patches/

> But as for the "page faults would use an extra lock to protect against
> each other" bit --- we already have another lock, the page table lock,
> which can be used in this way, so ANOTHER lock should be unnecessary.

Tomorrow I'll take a look at the various ->nopage
functions and do_swap_page to see if these functions
would be able to take simultaneous faults at the same
address (from multiple threads).  If not, either we'll
need to modify these functions, or we could add a (few?)
extra lock to prevent these functions from faulting at
the same address at the same time in multiple threads.

regards,

Rik
--
Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
http://www.conectiva.com/	http://distro.conectiva.com.br/

Re: changing mm->mmap_sem (was: Re: system call for process information?)

[Mike Galbraith]

On Sun, 18 Mar 2001, Rik van Riel wrote:

> On Fri, 16 Mar 2001, Stephen C. Tweedie wrote:
>
> > Right, I'm not suggesting removing that: making the mmap_sem
> > read/write is fine, but yes, we still need that semaphore.
>
> Initial patch (against 2.4.2-ac20) is available at
> http://www.surriel.com/patches/
>
> > But as for the "page faults would use an extra lock to protect against
> > each other" bit --- we already have another lock, the page table lock,
> > which can be used in this way, so ANOTHER lock should be unnecessary.
>
> Tomorrow I'll take a look at the various ->nopage
> functions and do_swap_page to see if these functions
> would be able to take simultaneous faults at the same
> address (from multiple threads).  If not, either we'll
> need to modify these functions, or we could add a (few?)
> extra lock to prevent these functions from faulting at
> the same address at the same time in multiple threads.

Hi Rik,

I gave this patch a try, and the initial results are extremely encouraging.
Not only do I have vmstat (SCHED_RR) info in realtime with zero delays :))
I also have a _nice_ throughput improvement.  There are some worrisome
warnings below along with the compile changes I made here, but for an
initial patch, things look pretty darn wonderful.

	Cheers,

	-Mike

--- ./include/linux/sched.h.org	Sun Mar 18 10:20:42 2001
+++ ./include/linux/sched.h	Sun Mar 18 10:27:48 2001
@@ -238,7 +238,7 @@
 	mm_users:	ATOMIC_INIT(2), 		\
 	mm_count:	ATOMIC_INIT(1), 		\
 	map_count:	1, 				\
-	mmap_sem:	__MUTEX_INITIALIZER(name.mmap_sem), \
+	mmap_sem:	__RWSEM_INITIALIZER(name.mmap_sem, RW_LOCK_BIAS), \
 	page_table_lock: SPIN_LOCK_UNLOCKED, 		\
 	mmlist:		LIST_HEAD_INIT(name.mmlist),	\
 }
--- ./include/linux/mm.h.org	Sun Mar 18 09:56:55 2001
+++ ./include/linux/mm.h	Sun Mar 18 10:27:59 2001
@@ -533,13 +533,13 @@
 	if (vma->vm_end - address > current->rlim[RLIMIT_STACK].rlim_cur ||
 	    ((vma->vm_mm->total_vm + grow) << PAGE_SHIFT) > current->rlim[RLIMIT_AS].rlim_cur)
 		return -ENOMEM;
-	spin_lock(&mm->page_table_lock);
+	spin_lock(&vma->vm_mm->page_table_lock);
 	vma->vm_start = address;
 	vma->vm_pgoff -= grow;
 	vma->vm_mm->total_vm += grow;
 	if (vma->vm_flags & VM_LOCKED)
 		vma->vm_mm->locked_vm += grow;
-	spin_unlock(&mm->page_table_lock);
+	spin_unlock(&vma->vm_mm->page_table_lock);
 	return 0;
 }

...
VFS: Mounted root (ext2 filesystem) readonly.
Freeing unused kernel memory: 196k freed
Adding Swap: 265064k swap-space (priority 2)
VM: Bad swap entry 00011e00
VM: Bad swap entry 00058d00
Unused swap offset entry in swap_dup 00058d00
Unused swap offset entry in swap_dup 00011e00
VM: Bad swap entry 00011e00
VM: Bad swap entry 00058d00
Unused swap offset entry in swap_dup 00058d00
VM: Bad swap entry 00058d00
Unused swap offset entry in swap_dup 00011e00
Unused swap offset entry in swap_dup 00058d00
VM: Bad swap entry 00011e00
VM: Bad swap entry 00058d00
Unused swap offset entry in swap_dup 00011e00
Unused swap offset entry in swap_dup 00058d00
VM: Bad swap entry 00011e00
VM: Bad swap entry 00058d00
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
Unused swap offset entry in swap_dup 006ef700
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_count 00011e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 008f4e00
Unused swap offset entry in swap_dup 006ef700
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 008f4e00
Unused swap offset entry in swap_dup 006ef700
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
Unused swap offset entry in swap_dup 00011e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 00011e00
Unused swap offset entry in swap_count 00011e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 006ef700
Unused swap offset entry in swap_dup 008f4e00
VM: Bad swap entry 006ef700
VM: Bad swap entry 008f4e00
Unused swap offset entry in swap_dup 008f4e00
Unused swap offset entry in swap_dup 006ef700

Re: changing mm->mmap_sem (was: Re: system call for process information?)

[Rik van Riel]

On Sun, 18 Mar 2001, Mike Galbraith wrote:

> I gave this patch a try, and the initial results are extremely encouraging.
> Not only do I have vmstat (SCHED_RR) info in realtime with zero delays :))
> I also have a _nice_ throughput improvement.  There are some worrisome
> warnings below along with the compile changes I made here, but for an
> initial patch, things look pretty darn wonderful.

	[snip compile fixes .. integrated]

> VFS: Mounted root (ext2 filesystem) readonly.
> Freeing unused kernel memory: 196k freed
> Adding Swap: 265064k swap-space (priority 2)
> VM: Bad swap entry 00011e00
> VM: Bad swap entry 00058d00
> Unused swap offset entry in swap_dup 00058d00
> Unused swap offset entry in swap_dup 00011e00
> VM: Bad swap entry 00011e00
> VM: Bad swap entry 00058d00

Heh, I guess do_swap_page isn't too happy when multiple threads
of the same program take a page fault at the same address at the
same time.

I take it you were testing something like mysql, jvm or apache2 ?

regards,

Rik
--
Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
http://www.conectiva.com/	http://distro.conectiva.com.br/

Re: changing mm->mmap_sem (was: Re: system call for process information?)

[Mike Galbraith]

On Sun, 18 Mar 2001, Rik van Riel wrote:

> > VFS: Mounted root (ext2 filesystem) readonly.
> > Freeing unused kernel memory: 196k freed
> > Adding Swap: 265064k swap-space (priority 2)
> > VM: Bad swap entry 00011e00
> > VM: Bad swap entry 00058d00
> > Unused swap offset entry in swap_dup 00058d00
> > Unused swap offset entry in swap_dup 00011e00
> > VM: Bad swap entry 00011e00
> > VM: Bad swap entry 00058d00
>
> Heh, I guess do_swap_page isn't too happy when multiple threads
> of the same program take a page fault at the same address at the
> same time.
>
> I take it you were testing something like mysql, jvm or apache2 ?

No, this was make -j30 bzImage.  (nscd was running though...)

	-Mike

Re: system call for process information?

[Rajiv Majumdar]

every process under Linux is assigned a *task_struct* structure which has
all the info about that process eg. scheduling, accounting, I/O and others.

rajiv




Guennadi Liakhovetski <g.liakhovetski@ragingbull.com> on 03/12/2001
10:38:46 PM

To:   linux-kernel@vger.kernel.org
cc:    (bcc: Rajiv Majumdar/CAL/NOTES)

Subject:  system call for process information?




Hello

I asked this question on kernel-newbies - no reply, hope to be luckier
here:-)

I need to collect some info on processes. One way is to read /proc
tree. But isn't there a system call (ioctl) for this? And what are those
task[], task_struct, etc. about?

Thanks
Guennadi
___

Dr. Guennadi V. Liakhovetski
Department of Applied Mathematics
University of Sheffield, U.K.
email: G.Liakhovetski@sheffield.ac.uk


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