linux-kernel - Re: [patch 1/3] raid5: offload stripe handle to workqueue

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Message-ID: <20130801020101.GB12580@kernel.org>
Date:	Thu, 1 Aug 2013 10:01:01 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
	neilb@...e.de, djbw@...com
Subject: Re: [patch 1/3] raid5: offload stripe handle to workqueue

On Wed, Jul 31, 2013 at 06:33:32AM -0400, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jul 31, 2013 at 09:24:34AM +0800, Shaohua Li wrote:
> > stripe is the work unit actually. As I said, if I queue a work for each stripe,
> > just queue_work() will make the system blast because of the pwq->pool->lock
> > contention. dispatching one work has another side effect that I can't add block
> 
> Hmmm.... I see.  I'm not familiar with the code base and could be
> missing something but how does the custom stripe dispatch queue
> synchronize?  Doesn't that make use of a lock anyway?  If so, how
> would scheduling separate work items be worse?  

It does have lock, but when a stripe is queued to handle, no lock is required.
So the workqueue lock will be high contended.

> Also, can you please
> elaborate the block plug part?

Basically I do:

blk_start_plug()
handle_stripe() //may dispatch request
blk_end_plug()

If only handle one stripe between block plug, the plug is useless, so I need
handle several stripes.

Thanks,
Shaohua
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