linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Date:	Tue, 12 Apr 2011 18:49:53 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	"hch@...radead.org" <hch@...radead.org>
CC:	Dave Chinner <david@...morbit.com>, NeilBrown <neilb@...e.de>,
	Mike Snitzer <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On 2011-04-12 18:44, hch@...radead.org wrote:
> On Tue, Apr 12, 2011 at 02:58:46PM +0200, Jens Axboe wrote:
>> Supposedly it's faster to do it inline rather than punt the dispatch.
>> But that may actually not be true, if you have multiple plugs going (and
>> thus multiple contenders for the queue lock on dispatch). So lets play
>> it safe and punt to kblockd, we can always revisit this later.
> 
> Note that this can be optimized further by adding a new helper that just
> queues up work on kblockd without taking the queue lock, e.g. adding a
> new
> 
> void blk_run_queue_async(struct request_queue *q)
> {
> 	if (likely(!blk_queue_stopped(q)))
> 		queue_delayed_work(kblockd_workqueue, &q->delay_work, 0);
> }
> 
> And replacing all
> 
> 	__blk_run_queue(q, true);
> 
> callers with that, at which point they won't need the queuelock any
> more.

I realize that, in fact it's already safe as long as you pass in 'true'
for __blk_run_queue(). Before I had rewritten it to move the running
out, so that makes the trick a little difficult. This afternoon I also
tested it and saw no noticable difference, but I'll probably just do it
anyway as it makes sense.

-- 
Jens Axboe

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