linux-kernel - Re: [PATCH] blk-mq: don't lose requests if a stopped queue restarts

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Message-ID: <5547C5BF.2030703@fb.com>
Date:	Mon, 4 May 2015 13:17:19 -0600
From:	Jens Axboe <axboe@...com>
To:	Shaohua Li <shli@...com>, <linux-kernel@...r.kernel.org>
CC:	<Kernel-team@...com>
Subject: Re: [PATCH] blk-mq: don't lose requests if a stopped queue restarts

On 05/02/2015 06:31 PM, Shaohua Li wrote:
> Normally if driver is busy to dispatch a request the logic is like below:
> block layer:					driver:
> 	__blk_mq_run_hw_queue
> a.						blk_mq_stop_hw_queue
> b.	rq add to ctx->dispatch
>
> later:
> 1.						blk_mq_start_hw_queue
> 2.	__blk_mq_run_hw_queue
>
> But it's possible step 1-2 runs between a and b. And since rq isn't in
> ctx->dispatch yet, step 2 will not run rq. The rq might get lost if
> there are no subsequent requests kick in.

Good catch! But the patch introduces a potentially never ending loop in 
__blk_mq_run_hw_queue(). Not sure how we can fully close it, but it 
might be better to punt the re-run after adding the requests back to the 
worker. That would turn a potential busy loop (until requests complete) 
into something with nicer behavior, at least. Ala

if (!test_bit(BLK_MQ_S_STOPPED, &hctx->state))
      kblockd_schedule_delayed_work_on(blk_mq_hctx_next_cpu(hctx),
                                         &hctx->run_work, 0);


-- 
Jens Axboe

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