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

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Message-ID: <87pnl0he9d.fsf@notabene.neil.brown.name>
Date:   Tue, 20 Aug 2019 10:27:58 +1000
From:   NeilBrown <neilb@...e.com>
To:     Jinpu Wang <jinpu.wang@...ud.ionos.com>
Cc:     Alexandr Iarygin <alexandr.iarygin@...ud.ionos.com>,
        Guoqing Jiang <guoqing.jiang@...ud.ionos.com>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        Neil F Brown <nfbrown@...e.com>,
        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 Fri, Aug 16 2019, Jinpu Wang wrote:

> 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?

Thanks for the reminder.  I've sent out the patch now.

NeilBrown


>
> Please let me know how can we move the fix forward.
>
> Thanks,
> Jack Wang

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