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, 07 Aug 2019 09:40:26 +1000
From:   NeilBrown <neilb@...e.com>
To:     Jinpu Wang <jinpu.wang@...ud.ionos.com>,
        Neil F Brown <nfbrown@...e.com>
Cc:     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 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.

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


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);

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