linux-kernel - Re: [PATCH] block: eliminate ELEVATOR_INSERT_REQUEUE (was: Re: elevator private data for REQ

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Message-ID: <20110330075752.GC17523@htj.dyndns.org>
Date:	Wed, 30 Mar 2011 09:57:52 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jeff Moyer <jmoyer@...hat.com>, Mike Snitzer <snitzer@...hat.com>,
	Jens Axboe <axboe@...nel.dk>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-kernel@...r.kernel.org, Chris Mason <chris.mason@...cle.com>
Subject: Re: [PATCH] block: eliminate ELEVATOR_INSERT_REQUEUE (was: Re:
 elevator private data for REQ_FLUSH)

On Wed, Mar 30, 2011 at 09:41:23AM +0200, Tejun Heo wrote:
> IIRC, at least in the requeue path, some drivers depend on front
> queueing for forward progress guarantee.  Forward progress for
> prepping is guaranteed by mempool (or something like that) and when
> the request is retried, it should stay at the front of the queue;
> otherwise, prepping can stall with prepped requests stuck behind
> unprepped ones.

After writing the above, I think the current flush implementation
actually is violating the above.  I think the problem is over-use of
front insertion.  Flush can just append to the dispatch queue.  Front
insertion should only be used by requeueing, really.

Thanks.

-- 
tejun
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