linux-kernel - Re: [PATCH] block: strip out locking optimization in put_io

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Message-ID: <20120208172313.GD19392@google.com>
Date:	Wed, 8 Feb 2012 09:23:13 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Shaohua Li <shaohua.li@...el.com>, Jens Axboe <axboe@...nel.dk>,
	Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>, mroos@...ux.ee
Subject: Re: [PATCH] block: strip out locking optimization in
 put_io_context()

On Wed, Feb 08, 2012 at 08:56:11AM -0800, Tejun Heo wrote:
> On Wed, Feb 08, 2012 at 08:49:20AM -0800, Tejun Heo wrote:
> > I'm still a bit lost on where the regression is coming from and
> > *suspecting* that queue_lock contention is making the reverse locking
> > behave much worse than expected, so I mostly wanted to take that out
> > and see what happens.
> 
> IOW, we can achieve about the same thing by adding another lock in
> request_queue.  The goal is using an inner lock for ioc clearing so
> that queue_lock doesn't have to be grabbed inside ioc lock.

Urgh....... forget about the above message.  My head is still booting.
We can't do this w/ per-queue lock as we don't have a way to traverse
the associated queues w/o holding the lock and we need read locking on
ioc exit path as we may recurse through elevator icq exit (it's not
about concurrency).  /me goes to brew coffee.

-- 
tejun
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