linux-kernel - Re: [RFC][PATCH 1/3] sched, block: Move unplug

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Date:	Wed, 22 Jun 2011 16:30:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jens Axboe <jaxboe@...ionio.com>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC][PATCH 1/3] sched, block: Move unplug

On Wed, 22 Jun 2011, Jens Axboe wrote:
> On 2011-06-22 15:53, Thomas Gleixner wrote:
> > On Wed, 22 Jun 2011, Jens Axboe wrote:
> > Is that a real problem or just a "we have no clue what might happen"
> > countermeasure? The plug list should not be magically refilled once
> > it's split off so this should not recurse endlessly, right? If it does
> > then we better fix it at the root cause of the problem and not by
> > adding some last resort band aid into the scheduler code.
> 
> It is supposedly a real problem, not just an inkling. It's not about
> recursing indefinitely, the plug is fairly bounded. But the IO dispatch
> path can be pretty deep, and if you hit that deep inside the reclaim or
> file system write path, then you get dangerously close. Dave Chinner
> posted some numbers in the 2.6.39-rc1 time frame showing how close we
> got.

Fair enough.
 
> We are talking past each other again. Flushing on going to sleep is
> needed. Placement of that call was pretty much left in the hands of the
> scheduler people. I personally don't care where it's put, as long as it
> does what is needed.

Ok. So we move it out and keep the from_scheduler flag so that code
does not go down the IO path from there.
 
Thanks,

	tglx
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