linux-kernel - Re: [PATCH] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

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Message-ID: <a25dbb6a-2ffe-a843-abc4-1a6608c17a49@kernel.dk>
Date:   Thu, 25 Oct 2018 10:25:57 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     ming.lei@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: ensure hctx to be ran on mapped cpu when issue
 directly

On 10/24/18 9:20 AM, Jianchao Wang wrote:
> When issue request directly and the task is migrated out of the
> original cpu where it allocates request, hctx could be ran on
> the cpu where it is not mapped. To fix this, insert the request
> if BLK_MQ_F_BLOCKING is set, check whether the current is mapped
> to the hctx and invoke __blk_mq_issue_directly under preemption
> disabled.
> 
> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
> ---
>  block/blk-mq.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index e3c39ea..0cdc306 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1717,6 +1717,12 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  {
>  	struct request_queue *q = rq->q;
>  	bool run_queue = true;
> +	blk_status_t ret;
> +
> +	if (hctx->flags & BLK_MQ_F_BLOCKING) {
> +		bypass_insert = false;
> +		goto insert;
> +	}

I'd do a prep patch that moves the insert logic out of this function,
and just have the caller do it by return BLK_STS_RESOURCE, for instance.
It's silly that we have that in both the caller and inside this function.

> @@ -1734,6 +1740,11 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	if (q->elevator && !bypass_insert)
>  		goto insert;
>  
> +	if (!cpumask_test_cpu(get_cpu(), hctx->cpumask)) {
> +		bypass_insert = false;
> +		goto insert;
> +	}

Should be fine to just do smp_processor_id() here, as we're inside
hctx_lock() here.

-- 
Jens Axboe