linux-kernel - Re: [PATCH 2/6] block: allow blk_execute_rq_nowait() to be called form IRQ context

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Date:	Fri, 21 Oct 2011 05:20:16 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, vgoyal@...hat.com, jgarzik@...ox.com,
	davem@...emloft.net, linux-kernel@...r.kernel.org,
	ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCH 2/6] block: allow blk_execute_rq_nowait() to be called
 form IRQ context

On Thu, Oct 20, 2011 at 08:56:36PM -0700, Tejun Heo wrote:
> Currently blk_execute_rq_nowait() directly calls __blk_run_queue() and
> thus can't be called from IRQ context.  This patch updates it to use
> blk_run_queue_async() instead.  This will be used to unexport
> elv_add_request().
> 
> This changes how queue is kicked after blk_execute_rq_nowait() but
> it's hardly a hot path and the effect shouldn't be noticeable.

It actually very much is a fasthpath for many of it's users, e.g. the
SCSI tape drivers, the OSD layer and the target scsi passthrough
backend.

I don't think blindly adding a context switch here without benchmarking
is doable.  Just add variants that do the workqueue dance or not.

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