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

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Date:	Mon, 11 Apr 2011 18:59:30 -0400
From:	"hch@...radead.org" <hch@...radead.org>
To:	NeilBrown <neilb@...e.de>
Cc:	"hch@...radead.org" <hch@...radead.org>,
	Mike Snitzer <snitzer@...hat.com>,
	Jens Axboe <jaxboe@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dm-devel@...hat.com, linux-raid@...r.kernel.org
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On Tue, Apr 12, 2011 at 07:14:28AM +1000, NeilBrown wrote:
> 
> My understanding of that was that the calling requirement of
> __elv_add_request is that the queue spinlock is held and that interrupts are
> disabled.
> So rather than possible enabling and disabling interrupts several times as
> different queue are handled, the code just disabled interrupts once, and
> then just take the spinlock once for each different queue.
> 
> The whole point of the change to plugging was to take locks less often.
> Disabling interrupts less often is presumably an analogous goal.
> 
> Though I agree that a comment would help.
> 
> 	q = NULL;
> +	/* Disable interrupts just once rather than using spin_lock_irq/sin_unlock_irq
> 	 * variants
> 	 */
> 	local_irq_save(flags);
> 
> 
> assuming my analysis is correct.

Your explanation does make sense to be now that you explain it.  I
didn't even thing of that variant before.

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