linux-kernel - Re: Bisected: Kernel 4.14 + has 3 times higher write IO latency than Kernel 4.4 with raid1

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Date:   Fri, 16 Aug 2019 10:10:43 +0200
From:   Jinpu Wang <jinpu.wang@...ud.ionos.com>
To:     NeilBrown <neilb@...e.com>
Cc:     Neil F Brown <nfbrown@...e.com>,
        Alexandr Iarygin <alexandr.iarygin@...ud.ionos.com>,
        Guoqing Jiang <guoqing.jiang@...ud.ionos.com>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        linux-kernel@...r.kernel.org,
        linux-raid <linux-raid@...r.kernel.org>
Subject: Re: Bisected: Kernel 4.14 + has 3 times higher write IO latency than
 Kernel 4.4 with raid1

On Wed, Aug 7, 2019 at 2:35 PM Jinpu Wang <jinpu.wang@...ud.ionos.com> wrote:
>
> On Wed, Aug 7, 2019 at 8:36 AM Jinpu Wang <jinpu.wang@...ud.ionos.com> wrote:
> >
> > On Wed, Aug 7, 2019 at 1:40 AM NeilBrown <neilb@...e.com> wrote:
> > >
> > > On Tue, Aug 06 2019, Jinpu Wang wrote:
> > >
> > > > On Tue, Aug 6, 2019 at 9:54 AM Jinpu Wang <jinpu.wang@...ud.ionos.com> wrote:
> > > >>
> > > >> On Tue, Aug 6, 2019 at 1:46 AM NeilBrown <neilb@...e.com> wrote:
> > > >> >
> > > >> > On Mon, Aug 05 2019, Jinpu Wang wrote:
> > > >> >
> > > >> > > Hi Neil,
> > > >> > >
> > > >> > > For the md higher write IO latency problem, I bisected it to these commits:
> > > >> > >
> > > >> > > 4ad23a97 MD: use per-cpu counter for writes_pending
> > > >> > > 210f7cd percpu-refcount: support synchronous switch to atomic mode.
> > > >> > >
> > > >> > > Do you maybe have an idea? How can we fix it?
> > > >> >
> > > >> > Hmmm.... not sure.
> > > >> Hi Neil,
> > > >>
> > > >> Thanks for reply, detailed result in line.
> > >
> > > Thanks for the extra testing.
> > > ...
> > > > [  105.133299] md md0 in_sync is 0, sb_flags 2, recovery 3, external
> > > > 0, safemode 0, recovery_cp 524288
> > > ...
> > >
> > > ahh - the resync was still happening.  That explains why set_in_sync()
> > > is being called so often.  If you wait for sync to complete (or create
> > > the array with --assume-clean) you should see more normal behaviour.
> > I've updated my tests accordingly, thanks for the hint.
> > >
> > > This patch should fix it.  I think we can do better but it would be more
> > > complex so no suitable for backports to -stable.
> > >
> > > Once you confirm it works, I'll send it upstream with a
> > > Reported-and-Tested-by from you.
> > >
> > > Thanks,
> > > NeilBrown
> >
> > Thanks a lot, Neil, my quick test show, yes, it fixed the problem for me.
> >
> > I will run more tests to be sure, will report back the test result.
> Hi Neil,
>
> I've run our regression tests with your patch, everything works fine
> as expected.
>
> So Reported-and-Tested-by: Jack Wang <jinpu.wang@...ud.ionos.com>
>
> Thank you for your quick fix.
>
> The patch should go to stable 4.12+

Hi Neil,

I hope you're doing well, just a soft ping? do you need further
testing from my side?

Please let me know how can we move the fix forward.

Thanks,
Jack Wang