linux-kernel - Re: [REGRESSION] 5-10% increase in IO latencies with nohz balance patch

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Date:   Wed, 22 Dec 2021 16:07:35 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Thorsten Leemhuis <regressions@...mhuis.info>,
        Josef Bacik <josef@...icpanda.com>
Cc:     peterz@...radead.org, vincent.guittot@...aro.org,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, guro@...com, clm@...com
Subject: Re: [REGRESSION] 5-10% increase in IO latencies with nohz balance patch


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?

> Ciao, Thorsten
>