linux-kernel - Re: [tip:sched/urgent] sched: Fix rq->nr

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Message-ID: <CADZ9YHiG56TCORVySHYjnhLBSsVi=OaMRmBq-_3yDTQCGnEbYw@mail.gmail.com>
Date:	Fri, 27 Jan 2012 20:11:15 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, kosaki.motohiro@...il.com,
	mingo@...e.hu
Subject: Re: [tip:sched/urgent] sched: Fix rq->nr_uninterruptible update race

On Fri, Jan 27, 2012 at 2:19 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Fri, 2012-01-27 at 11:20 +0600, Rakib Mullick wrote:
>> On Fri, Jan 27, 2012 at 2:25 AM, tip-bot for Peter Zijlstra
>> <a.p.zijlstra@...llo.nl> wrote:

>> Why would we want to avoid nr_uninterruptible accounting?
>> nr_uninterruptible has impact on load calculation, we might not get
>> the proper load weight if we don't account it. isn't it?
>
> Read again ;-)
>
Wasn't enough! I had to use paper and pen ;-)

> sched_setscheduler() did:
>
>  deactivate_task(); // remove it from the queue
>
>  // change tasks's scheduler paramater
>
>  activate_task(); // queue it in the new place
>
> it is invariant wrt nr_uninterruptible but does include the
> nr_uinterruptile accounting logic.
>
> Now Kosaki-San noticed that if the task manages to change its ->state at
> an inopportune moment (right between the dequeue and enqueue) we'll get
> screwy nr_uninterruptible accounting.

I got your point and yes, we'll get screwy nr_uninterruptible
accounting if we call {activate,deactivate}_task(). We need to avoid
nr_uninterruptible accounting.

Thanks,
Rakib
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