linux-kernel - Re: [PATCH 2/8] blk-mq: protect completion path with RCU

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Message-ID: <1515514359.2721.9.camel@wdc.com>
Date:   Tue, 9 Jan 2018 16:12:40 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "jbacik@...com" <jbacik@...com>, "tj@...nel.org" <tj@...nel.org>,
        "jack@...e.cz" <jack@...e.cz>, "clm@...com" <clm@...com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
CC:     "kernel-team@...com" <kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>
Subject: Re: [PATCH 2/8] blk-mq: protect completion path with RCU

On Mon, 2018-01-08 at 11:15 -0800, Tejun Heo wrote:
> Currently, blk-mq protects only the issue path with RCU.  This patch
> puts the completion path under the same RCU protection.  This will be
> used to synchronize issue/completion against timeout by later patches,
> which will also add the comments.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---
>  block/blk-mq.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ddc9261..6741c3e 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -584,11 +584,16 @@ static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx)
>  void blk_mq_complete_request(struct request *rq)
>  {
>  	struct request_queue *q = rq->q;
> +	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
> +	int srcu_idx;
>  
>  	if (unlikely(blk_should_fake_timeout(q)))
>  		return;
> +
> +	hctx_lock(hctx, &srcu_idx);
>  	if (!blk_mark_rq_complete(rq))
>  		__blk_mq_complete_request(rq);
> +	hctx_unlock(hctx, srcu_idx);
>  }
>  EXPORT_SYMBOL(blk_mq_complete_request);

Hello Tejun,

I'm concerned about the additional CPU cycles needed for the new blk_mq_map_queue()
call, although I know this call is cheap. Would the timeout code really get that
much more complicated if the hctx_lock() and hctx_unlock() calls would be changed
into rcu_read_lock() and rcu_read_unlock() calls? Would it be sufficient if
"if (has_rcu) synchronize_rcu();" would be changed into "synchronize_rcu();" in
blk_mq_timeout_work()?

Thanks,

Bart.