From mboxrd@z Thu Jan  1 00:00:00 1970
From: NeilBrown <neilb@suse.com>
Subject: Re: Bisected: Kernel 4.14 + has 3 times higher write IO latency than Kernel 4.4 with raid1
Date: Tue, 06 Aug 2019 09:46:27 +1000
Message-ID: <87h86vjhv0.fsf@notabene.neil.brown.name>
References: <CAMGffEkotpvVz8FA78vNFh0qZv3kEMNrXXfVPEUC=MhH0pMCZA@mail.gmail.com> <0a83fde3-1a74-684c-0d70-fb44b9021f96@molgen.mpg.de> <CAMGffE=_kPoBmSwbxvrqdqbhpR5Cu2Vbe4ArGqm9ns9+iVEH_g@mail.gmail.com> <CAMGffEkcXcQC+kjwdH0iVSrFDk-o+dp+b3Q1qz4z=R=6D+QqLQ@mail.gmail.com>
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To: Jinpu Wang <jinpu.wang@cloud.ionos.com>, linux-raid <linux-raid@vger.kernel.org>
Cc: Alexandr Iarygin <alexandr.iarygin@cloud.ionos.com>, Guoqing Jiang <guoqing.jiang@cloud.ionos.com>, Paul Menzel <pmenzel@molgen.mpg.de>, linux-kernel@vger.kernel.org
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On Mon, Aug 05 2019, Jinpu Wang wrote:

> Hi Neil,
>
> For the md higher write IO latency problem, I bisected it to these commit=
s:
>
> 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.

My guess is that the set_in_sync() call from md_check_recovery()
is taking a long time, and is being called too often.

Could you try two experiments please.

1/ set  /sys/block/md0/md/safe_mode_delay=20
   to 20 or more.  It defaults to about 0.2.

2/ comment out the call the set_in_sync() in md_check_recovery().

Then run the least separately after each of these changes.

I the second one makes a difference, I'd like to know how often it gets
called - and why.  The test
	if ( ! (
		(mddev->sb_flags & ~ (1<<MD_SB_CHANGE_PENDING)) ||
		test_bit(MD_RECOVERY_NEEDED, &mddev->recovery) ||
		test_bit(MD_RECOVERY_DONE, &mddev->recovery) ||
		(mddev->external =3D=3D 0 && mddev->safemode =3D=3D 1) ||
		(mddev->safemode =3D=3D 2
		 && !mddev->in_sync && mddev->recovery_cp =3D=3D MaxSector)
		))
		return;

should normally return when doing lots of IO - I'd like to know
which condition causes it to not return.

Thanks,
NeilBrown

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