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:   Wed, 7 Aug 2019 08:36:51 +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 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.

Regards,
Jack Wang

>
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 24638ccedce4..624cf1ac43dc 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -8900,6 +8900,7 @@ void md_check_recovery(struct mddev *mddev)
>
>         if (mddev_trylock(mddev)) {
>                 int spares = 0;
> +               bool try_set_sync = mddev->safemode != 0;
>
>                 if (!mddev->external && mddev->safemode == 1)
>                         mddev->safemode = 0;
> @@ -8945,7 +8946,7 @@ void md_check_recovery(struct mddev *mddev)
>                         }
>                 }
>
> -               if (!mddev->external && !mddev->in_sync) {
> +               if (try_set_sync && !mddev->external && !mddev->in_sync) {
>                         spin_lock(&mddev->lock);
>                         set_in_sync(mddev);
>                         spin_unlock(&mddev->lock);