linux-kernel - Re: [GIT PULL] Core block IO bits for 2.6.39

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Date:	Sun, 27 Mar 2011 14:00:57 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [GIT PULL] Core block IO bits for 2.6.39

On 2011-03-27 13:49, Avi Kivity wrote:
> On 03/24/2011 03:43 PM, Jens Axboe wrote:
>> Hi Linus,
>>
>> This is the main pull request for the block IO layer and friends for
>> 2.6.39.
>>
>> There are two major things in this tree:
>>
>> - The removal of the per-device plugging state for disks. On fast
>>    devices, it ended up hammering the queue lock quite hard. The new
>>    scheme puts the plugging state on the stack and allows an IO submitter
>>    to finish his batch of IO before pushing it to the queue. Once that
>>    push starts, we'll insert/merge with the existing queue.
>>
>>    A pointer to this plugging context is stored in the task structure. If
>>    a task ends up blocking before it has submitted it's IO (usual cause
>>    would be memory allocation of some sort), the plugged list is
>>    auto-submitted before the task goes to sleep.
> 
> This is the fourth "do something if preempted" hook (the other three are 
> kvm, cmwq, and perf).  Why not use sched notifiers for this?

It's a 'if preempted', it's 'if going to sleep'. Two issues with the
preempt notifiers for this, I did look into it:

- Not unconditionally available. Not a big problem, we could just do
  that.

- Sched out is called under the runqueue lock, and it's not safe to drop
  it.

But it's not a bad point, it would be nice if we could unify all these
use cases and provide a notifier that works for all.

-- 
Jens Axboe

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