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From:   Song Liu <song@kernel.org>
Date:   Thu, 7 Dec 2023 10:24:38 -0800
Message-ID: <CAPhsuW65GoX1Sewndu9ViUYTCKgLxy-VhbPkkXPBO6P2g-1UrQ@mail.gmail.com>
Subject: Re: [PATCH v2] md: split MD_RECOVERY_NEEDED out of mddev_resume
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     yukuai3@huawei.com, pmenzel@molgen.mpg.de,
        janpieter.sollie@edpnet.be, linux-raid@vger.kernel.org,
        linux-kernel@vger.kernel.org, yi.zhang@huawei.com,
        yangerkun@huawei.com
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On Wed, Dec 6, 2023 at 6:08=E2=80=AFPM Yu Kuai <yukuai1@huaweicloud.com> wr=
ote:
>
> From: Yu Kuai <yukuai3@huawei.com>
>
> New mddev_resume() calls are added to synchronize IO with array
> reconfiguration, however, this introduces a performance regression while
> adding it in md_start_sync():
>
> 1) someone sets MD_RECOVERY_NEEDED first;
> 2) daemon thread grabs reconfig_mutex, then clears MD_RECOVERY_NEEDED and
>    queues a new sync work;
> 3) daemon thread releases reconfig_mutex;
> 4) in md_start_sync
>    a) check that there are spares that can be added/removed, then suspend
>       the array;
>    b) remove_and_add_spares may not be called, or called without really
>       add/remove spares;
>    c) resume the array, then set MD_RECOVERY_NEEDED again!
>
> Loop between 2 - 4, then mddev_suspend() will be called quite often, for
> consequence, normal IO will be quite slow.
>
> Fix this problem by don't set MD_RECOVERY_NEEDED again in md_start_sync()=
,
> hence the loop will be broken.
>
> Fixes: bc08041b32ab ("md: suspend array in md_start_sync() if array need =
reconfiguration")
> Suggested-by: Song Liu <song@kernel.org>
> Reported-by: Janpieter Sollie <janpieter.sollie@edpnet.be>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=3D218200
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Thanks for the fix! I added a comment and applied it to md-fixes.

Song