Re: [PATCH 2/4] sched: Fix race in idle_balance()

From: Daniel Lezcano
Date: Fri Jan 17 2014 - 08:44:14 EST

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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