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

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Message-ID: <20110412164417.GA13890@infradead.org>
Date:	Tue, 12 Apr 2011 12:44:17 -0400
From:	"hch@...radead.org" <hch@...radead.org>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Dave Chinner <david@...morbit.com>,
	"hch@...radead.org" <hch@...radead.org>, 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 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.

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