linux-kernel - Re: [PATCH 2/4] sched: Fix race in idle

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Date:	Fri, 17 Jan 2014 14:44:00 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linaro-kernel@...ts.linaro.org, alex.shi@...aro.org
Subject: Re: [PATCH 2/4] sched: Fix race in idle_balance()

On 01/17/2014 02:33 PM, Peter Zijlstra wrote:
> On Fri, Jan 17, 2014 at 10:04:02AM +0100, Daniel Lezcano wrote:
>> The scheduler main function 'schedule()' checks if there are no more tasks
>> on the runqueue. Then it checks if a task should be pulled in the current
>> runqueue in idle_balance() assuming it will go to idle otherwise.
>>
>> But the idle_balance() releases the rq->lock in order to lookup in the sched
>> domains and takes the lock again right after. That opens a window where
>> another cpu may put a task in our runqueue, so we won't go to idle but
>> we have filled the idle_stamp, thinking we will.
>>
>> This patch closes the window by checking if the runqueue has been modified
>> but without pulling a task after taking the lock again, so we won't go to idle
>> right after in the __schedule() function.
>
> Did you actually observe this or was it found by reading the code?

When I tried to achieve what is doing the patch 4/4, I was falling in 
the BUG() (comment in patch 4/4). So I did some tests and checked that 
we enter idle_balance() with nr_running == 0 but we exit with nr_running 
 > 0 and pulled_task == 0.


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