linux-kernel - Re: [PATCH] block: Make rq_affinity = 1 work as expected.

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Date:	Fri, 5 Aug 2011 14:26:33 -0700
From:	"Williams, Dan J" <dan.j.williams@...el.com>
To:	Shaohua Li <shli@...nel.org>
Cc:	Tao Ma <tm@....ma>, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Roland Dreier <roland@...estorage.com>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH] block: Make rq_affinity = 1 work as expected.

On Thu, Aug 4, 2011 at 10:12 PM, Shaohua Li <shli@...nel.org> wrote:
> 2011/8/5 Tao Ma <tm@....ma>:
>> From: Tao Ma <boyu.mt@...bao.com>
>>
>> Commit 5757a6d76c introduced a new rq_affinity = 2 so as to make
>> the request completed in the __make_request cpu. But it makes the
>> old rq_affinity = 1 not work any more. The root cause is that
>> if the 'cpu' and 'req->cpu' is in the same group and cpu != req->cpu,
>> ccpu will be the same as group_cpu, so the completion will be
>> excuted in the 'cpu' not 'group_cpu'.
>>
>> This patch fix problem by simpling removing group_cpu and the codes
>> are more explicit now. If ccpu == cpu, we complete in cpu, otherwise
>> we raise_blk_irq to ccpu.
> good catch. This changed old behavior and can cause more lock contention.
> and if user doesn't care about lock contention, he can use rq_affinity = 2
>

Indeed it does change behavior of rq_affinity=1, apologies.

Reviewed-by: Dan Williams <dan.j.williams@...el.com>
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