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    SubjectRe: [REGRESSION] 5-10% increase in IO latencies with nohz balance patch
    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!

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