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

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Date:   Mon, 19 Jun 2023 12:26:59 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     aligrudi@...il.com, song@...nel.org, 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()

Dear Yu,


Thank you for your patch. Some minor nits from my side, you can also ignore.

Am 18.06.23 um 16:25 schrieb Yu Kuai:
> 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:

… one place. For example, with

> 
> fio -direct=1 -rw=write/randwrite -iodepth=1 ...
> 
> Then plug and unplug will be called for each io, then wake_up() from

Maybe:

     fio -direct=1 -rw=write/randwrite -iodepth=1 ...

plug und unplug are called for each io, then …

> raid10_unplug() will cause lock contention as well.

Maybe paste the perf command and output?

> Avoid this contention by using wake_up_barrier() instead of wake_up(),
> where spin_lock is not held while waitqueue is empty.

It’d be great if you added also the test results to the commit message.

> 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/
> 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;

Reviewed-by: Paul Menzel <pmenzel@...gen.mpg.de>


Kind regards,

Paul