linux-kernel - Re: [PATCH] raid10: avoid spin_lock from fastpath from raid10

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Date:   Tue, 20 Jun 2023 10:38:20 -0700
From:   Song Liu <song@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     aligrudi@...il.com, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH] raid10: avoid spin_lock from fastpath from raid10_unplug()

On Sat, Jun 17, 2023 at 11:26 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> From: Yu Kuai <yukuai3@...wei.com>
>
> Commit 0c0be98bbe67 ("md/raid10: prevent unnecessary calls to wake_up()
> in fast path") missed one place, for example, while testing with:
>
> fio -direct=1 -rw=write/randwrite -iodepth=1 ...
>
> Then plug and unplug will be called for each io, then wake_up() from
> raid10_unplug() will cause lock contention as well.
>
> Avoid this contention by using wake_up_barrier() instead of wake_up(),
> where spin_lock is not held while waitqueue is empty.
>
> By the way, in this scenario, each blk_plug_cb() will be allocated and
> freed for each io, this seems need to be optimized as well.
>
> Reported-and-tested-by: Ali Gholami Rudi <aligrudi@...il.com>
> Link: https://lore.kernel.org/all/20231606122233@laper.mirepesht/

I think ./scripts/checkpatch.pl would recommend using "Closes" instead of
"Link" after "Reported-by". Please follow this pattern.

Thanks,
Song

> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  drivers/md/raid10.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index d0de8c9fb3cf..fbaaa5e05edc 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1118,7 +1118,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
>                 spin_lock_irq(&conf->device_lock);
>                 bio_list_merge(&conf->pending_bio_list, &plug->pending);
>                 spin_unlock_irq(&conf->device_lock);
> -               wake_up(&conf->wait_barrier);
> +               wake_up_barrier(conf);
>                 md_wakeup_thread(mddev->thread);
>                 kfree(plug);
>                 return;
> @@ -1127,7 +1127,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
>         /* we aren't scheduling, so we can do the write-out directly. */
>         bio = bio_list_get(&plug->pending);
>         raid1_prepare_flush_writes(mddev->bitmap);
> -       wake_up(&conf->wait_barrier);
> +       wake_up_barrier(conf);
>
>         while (bio) { /* submit pending writes */
>                 struct bio *next = bio->bi_next;
> --
> 2.39.2
>