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Date:   Fri, 18 Feb 2022 12:00:41 +0100
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Subject: Re: [REGRESSION] 5-10% increase in IO latencies with nohz balance
 patch
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To:     Roman Gushchin <guro@fb.com>,
        Valentin Schneider <valentin.schneider@arm.com>
Cc:     Josef Bacik <josef@toxicpanda.com>, peterz@infradead.org,
        vincent.guittot@linaro.org, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org,
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From:   Thorsten Leemhuis <regressions@leemhuis.info>
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Hi, this is your Linux kernel regression tracker speaking. Top-posting
for once, to make this easy accessible to everyone.

FWIW, this is a gentle reminder that I'm still tracking this regression.
Afaics nothing happened in the last few weeks.

If the discussion continued somewhere else, please let me know; you can
do this directly or simply tell my regression tracking bot yourself by
sending a reply to this mail with a paragraph containing a regzbot
command like "#regzbot monitor
https://lore.kernel.org/r/some_msgi@example.com/"

If you think there are valid reasons to drop this regressions from the
tracking, let me know; you can do this directly or simply tell my
regression tracking bot yourself by sending a reply to this mail with a
paragraph containing a regzbot command like "#regzbot invalid: Some
explanation" (without the quotes).

Anyway: I'm putting it on back burner now to reduce the noise, as this
afaics is less important than other regressions:

#regzbot backburner: Culprit is hard to track down
#regzbot poke

You likely get two more mails like this after the next two merge
windows, then I'll drop it if I don't here anything back.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I'm getting a lot of
reports on my table. I can only look briefly into most of them and lack
knowledge about most of the areas they concern. I thus unfortunately
will sometimes get things wrong or miss something important. I hope
that's not the case here; if you think it is, don't hesitate to tell me
in a public reply, it's in everyone's interest to set the public record
straight.


On 13.01.22 17:57, Roman Gushchin wrote:
> On Thu, Jan 13, 2022 at 04:41:57PM +0000, Valentin Schneider wrote:
>> On 03/01/22 11:16, Josef Bacik wrote:
>>> On Wed, Dec 22, 2021 at 04:07:35PM +0000, Valentin Schneider wrote:
>>>>
>>>> Hi,
>>>>
>>>> On 22/12/21 13:42, Thorsten Leemhuis wrote:
>>>>> What's the status here? Just wondering, because there hasn't been any
>>>>> activity in this thread since 11 days and the festive season is upon us.
>>>>>
>>>>> Was the discussion moved elsewhere? Or is this still a mystery? And if
>>>>> it is: how bad is it, does it need to be fixed before Linus releases 5.16?
>>>>>
>>>>
>>>> I got to the end of bisect #3 yesterday, the incriminated commit doesn't
>>>> seem to make much sense but I've just re-tested it and there is a clear
>>>> regression between that commit and its parent (unlike bisect #1 and #2):
>>>>
>>>> 2127d22509aec3a83dffb2a3c736df7ba747a7ce mm, slub: fix two bugs in slab_debug_trace_open()
>>>> write_clat_ns_p99     195395.92     199638.20      4797.01    2.17%
>>>> write_iops             17305.79      17188.24       250.66   -0.68%
>>>>
>>>> write_clat_ns_p99     195543.84     199996.70      5122.88    2.28%
>>>> write_iops             17300.61      17241.86       251.56   -0.34%
>>>>
>>>> write_clat_ns_p99     195543.84     200724.48      5122.88    2.65%
>>>> write_iops             17300.61      17246.63       251.56   -0.31%
>>>>
>>>> write_clat_ns_p99     195543.84     200445.41      5122.88    2.51%
>>>> write_iops             17300.61      17215.47       251.56   -0.49%
>>>>
>>>> 6d2aec9e123bb9c49cb5c7fc654f25f81e688e8c mm/mempolicy: do not allow illegal MPOL_F_NUMA_BALANCING | MPOL_LOCAL in mbind() 
>>>> write_clat_ns_p99     195395.92     197942.30      4797.01    1.30%
>>>> write_iops             17305.79      17246.56       250.66   -0.34%
>>>>
>>>> write_clat_ns_p99     195543.84     196183.92      5122.88    0.33%
>>>> write_iops             17300.61      17310.33       251.56    0.06%
>>>>
>>>> write_clat_ns_p99     195543.84     196990.71      5122.88    0.74%
>>>> write_iops             17300.61      17346.32       251.56    0.26%
>>>>
>>>> write_clat_ns_p99     195543.84     196362.24      5122.88    0.42%
>>>> write_iops             17300.61      17315.71       251.56    0.09%
>>>>
>>>> It's pure debug stuff and AFAICT is a correct fix...
>>>> @Josef, could you test that on your side?
>>>
>>> Sorry, holidays and all that.  I see 0 difference between the two commits, and
>>> no regression from baseline.  It'll take me a few days to recover from the
>>> holidays, but I'll put some more effort into actively debugging wtf is going on
>>> here on my side since we're all having trouble pinning down what's going
>>> on.
>>
>> Humph, that's unfortunate... I just came back from my holidays, so I'll be
>> untangling my inbox for the next few days. Do keep us posted!
> 
> I'm trying to bisect it independently and make sense of it too, thanks to Josef
> for providing me a test setup. From the very first data I've got yesterday,
> the only thing I can say the data is very noisy and I'm not totally convinced
> that the regression is coming from the patch which was blamed initially.
> 
> I hope to make more progress today/tomorrow, will keep you updated.
> 
> Thanks!
>