Re: NFS server regression in kernel 5.13 (tested w/ 5.13.9)

[Chuck Lever III <chuck.lever@oracle.com>]

> On Sep 4, 2021, at 1:41 PM, Mike Javorski <mike.javorski@gmail.com> wrote:
> 
> Hi Chuck.
> 
> I noticed that you sent in the 5.15 pull request but Neil's fix
> (e38b3f20059426a0adbde014ff71071739ab5226 in your tree) missed the
> pull and thus the fix isn't going to be backported to 5.14 in the near
> term. Is there another 5.15 pull planned in the not too distant future
> so this will get flagged for back-porting,

Yes. The final version of Neil’s patch was just a little late for the initial v5.15 NFSD pull request (IMO) so it’s queued for the next PR, probably this week.


> or do I need to reach out to someone to expressly pull it into 5.14? If the latter, can you
> point me in the right direction of who to ask (I assume it's someone
> other than Greg KH)?
> 
> Thanks
> 
> - mike
> 
> 
>> On Sat, Aug 28, 2021 at 11:23 AM Chuck Lever III <chuck.lever@oracle.com> wrote:
>> 
>> 
>> 
>>>> On Aug 27, 2021, at 11:22 PM, Mike Javorski <mike.javorski@gmail.com> wrote:
>>> 
>>> I had some time this evening (and the kernel finally compiled), and
>>> wanted to get this tested.
>>> 
>>> The TL;DR:  Both patches are needed
>>> 
>>> Below are the test results from my replication of Neil's test. It is
>>> readily apparent that both the 5.13.13 kernel AND the 5.13.13 kernel
>>> with the 82011c80b3ec fix exhibit the randomness in read times that
>>> were observed. The 5.13.13 kernel with both the 82011c80b3ec and
>>> f6e70aab9dfe fixes brings the performance back in line with the
>>> 5.12.15 kernel which I tested as a baseline.
>>> 
>>> Please forgive the inconsistency in sample counts. This was running as
>>> a while loop, and I just let it go long enough that the behavior was
>>> consistent. Only change to the VM between tests was the different
>>> kernel + a reboot. The testing PC had a consistent workload during the
>>> entire set of tests.
>>> 
>>> Test 0: 5.13.10 (base kernel in VM image, just for kicks)
>>> ==================================================
>>> Samples 30
>>> Min 6.839
>>> Max 19.998
>>> Median 9.638
>>> 75-P 10.898
>>> 95-P 12.939
>>> 99-P 18.005
>>> 
>>> Test 1: 5.12.15 (known good)
>>> ==================================================
>>> Samples 152
>>> Min 1.997
>>> Max 2.333
>>> Median 2.171
>>> 75-P 2.230
>>> 95-P 2.286
>>> 99-P 2.312
>>> 
>>> Test 2: 5.13.13 (known bad)
>>> ==================================================
>>> Samples 42
>>> Min 3.587
>>> Max 15.803
>>> Median 6.039
>>> 75-P 6.452
>>> 95-P 10.293
>>> 99-P 15.540
>>> 
>>> Test 3: 5.13.13 + 82011c80b3ec fix
>>> ==================================================
>>> Samples 44
>>> Min 4.309
>>> Max 37.040
>>> Median 6.615
>>> 75-P 10.224
>>> 95-P 19.516
>>> 99-P 36.650
>>> 
>>> Test 4: 5.13.13 + 82011c80b3ec fix + f6e70aab9dfe fix
>>> ==================================================
>>> Samples 131
>>> Min 2.013
>>> Max 2.397
>>> Median 2.169
>>> 75-P 2.211
>>> 95-P 2.283
>>> 99-P 2.348
>>> 
>>> I am going to run the kernel w/ both fixes over the weekend, but
>>> things look good at this point.
>>> 
>>> - mike
>> 
>> I've targeted Neil's fix for the first 5.15-rc NFSD pull request.
>> I'd like to have Mel's Reviewed-by or Acked-by, though.
>> 
>> I will add a Fixes: tag if Neil doesn't repost (no reason to at
>> this point) so the fix should get backported automatically to
>> recent stable kernels.
>> 
>> 
>>> On Fri, Aug 27, 2021 at 4:49 PM Chuck Lever III <chuck.lever@oracle.com> wrote:
>>>> 
>>>> 
>>>>> On Aug 27, 2021, at 6:00 PM, Mike Javorski <mike.javorski@gmail.com> wrote:
>>>>> 
>>>>> OK, an update. Several hours of spaced out testing sessions and the
>>>>> first patch seems to have resolved the issue. There may be a very tiny
>>>>> bit of lag that still occurs when opening/processing new files, but so
>>>>> far on this kernel I have not had any multi-second freezes. I am still
>>>>> waiting on the kernel with Neil's patch to compile (compiling on this
>>>>> underpowered server so it's taking several hours), but I think the
>>>>> testing there will just be to see if I can show it works still, and
>>>>> then to try and test in a memory constrained VM. To see if I can
>>>>> recreate Neil's experiment. Likely will have to do this over the
>>>>> weekend given the kernel compile delay + fiddling with a VM.
>>>> 
>>>> Thanks for your testing!
>>>> 
>>>> 
>>>>> Chuck: I don't mean to overstep bounds, but is it possible to get that
>>>>> patch pulled into 5.13 stable? That may help things for several people
>>>>> while 5.14 goes through it's shakedown in archlinux prior to release.
>>>> 
>>>> The patch had a Fixes: tag, so it should get automatically backported
>>>> to every kernel that has the broken commit. If you don't see it in
>>>> a subsequent 5.13 stable kernel, you are free to ask the stable
>>>> maintainers to consider it.
>>>> 
>>>> 
>>>>> - mike
>>>>> 
>>>>> On Fri, Aug 27, 2021 at 10:07 AM Mike Javorski <mike.javorski@gmail.com> wrote:
>>>>>> 
>>>>>> Chuck:
>>>>>> I just booted a 5.13.13 kernel with your suggested patch. No freezes
>>>>>> on the first test, but that sometimes happens so I will let the server
>>>>>> settle some and try it again later in the day (which also would align
>>>>>> with Neil's comment on memory fragmentation being a contributor).
>>>>>> 
>>>>>> Neil:
>>>>>> I have started a compile with the above kernel + your patch to test
>>>>>> next unless you or Chuck determine that it isn't needed, or that I
>>>>>> should test both patches discreetly. As the above is already merged to
>>>>>> 5.14 it seemed logical to just add your patch on top.
>>>>>> 
>>>>>> I will also try to set up a vm to test your md5sum scenario with the
>>>>>> various kernels since it's a much faster thing to test.
>>>>>> 
>>>>>> - mike
>>>>>> 
>>>>>> On Fri, Aug 27, 2021 at 7:13 AM Chuck Lever III <chuck.lever@oracle.com> wrote:
>>>>>>> 
>>>>>>> 
>>>>>>>> On Aug 27, 2021, at 3:14 AM, NeilBrown <neilb@suse.de> wrote:
>>>>>>>> 
>>>>>>>> Subject: [PATCH] SUNRPC: don't pause on incomplete allocation
>>>>>>>> 
>>>>>>>> alloc_pages_bulk_array() attempts to allocate at least one page based on
>>>>>>>> the provided pages, and then opportunistically allocates more if that
>>>>>>>> can be done without dropping the spinlock.
>>>>>>>> 
>>>>>>>> So if it returns fewer than requested, that could just mean that it
>>>>>>>> needed to drop the lock.  In that case, try again immediately.
>>>>>>>> 
>>>>>>>> Only pause for a time if no progress could be made.
>>>>>>> 
>>>>>>> The case I was worried about was "no pages available on the
>>>>>>> pcplist", in which case, alloc_pages_bulk_array() resorts
>>>>>>> to calling __alloc_pages() and returns only one new page.
>>>>>>> 
>>>>>>> "No progess" would mean even __alloc_pages() failed.
>>>>>>> 
>>>>>>> So this patch would behave essentially like the
>>>>>>> pre-alloc_pages_bulk_array() code: call alloc_page() for
>>>>>>> each empty struct_page in the array without pausing. That
>>>>>>> seems correct to me.
>>>>>>> 
>>>>>>> 
>>>>>>> I would add
>>>>>>> 
>>>>>>> Fixes: f6e70aab9dfe ("SUNRPC: refresh rq_pages using a bulk page allocator")
>>>>>>> 
>>>>>>> 
>>>>>>>> Signed-off-by: NeilBrown <neilb@suse.de>
>>>>>>>> ---
>>>>>>>> net/sunrpc/svc_xprt.c | 7 +++++--
>>>>>>>> 1 file changed, 5 insertions(+), 2 deletions(-)
>>>>>>>> 
>>>>>>>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>>>>>>>> index d66a8e44a1ae..99268dd95519 100644
>>>>>>>> --- a/net/sunrpc/svc_xprt.c
>>>>>>>> +++ b/net/sunrpc/svc_xprt.c
>>>>>>>> @@ -662,7 +662,7 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
>>>>>>>> {
>>>>>>>>    struct svc_serv *serv = rqstp->rq_server;
>>>>>>>>    struct xdr_buf *arg = &rqstp->rq_arg;
>>>>>>>> -     unsigned long pages, filled;
>>>>>>>> +     unsigned long pages, filled, prev;
>>>>>>>> 
>>>>>>>>    pages = (serv->sv_max_mesg + 2 * PAGE_SIZE) >> PAGE_SHIFT;
>>>>>>>>    if (pages > RPCSVC_MAXPAGES) {
>>>>>>>> @@ -672,11 +672,14 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
>>>>>>>>            pages = RPCSVC_MAXPAGES;
>>>>>>>>    }
>>>>>>>> 
>>>>>>>> -     for (;;) {
>>>>>>>> +     for (prev = 0;; prev = filled) {
>>>>>>>>            filled = alloc_pages_bulk_array(GFP_KERNEL, pages,
>>>>>>>>                                            rqstp->rq_pages);
>>>>>>>>            if (filled == pages)
>>>>>>>>                    break;
>>>>>>>> +             if (filled > prev)
>>>>>>>> +                     /* Made progress, don't sleep yet */
>>>>>>>> +                     continue;
>>>>>>>> 
>>>>>>>>            set_current_state(TASK_INTERRUPTIBLE);
>>>>>>>>            if (signalled() || kthread_should_stop()) {
>>>>>>> 
>>>>>>> --
>>>>>>> Chuck Lever
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>> 
>>>> --
>>>> Chuck Lever
>>>> 
>>>> 
>>>> 
>> 
>> --
>> Chuck Lever
>> 
>> 
>>