Commit 791c9e02 authored Feb 18, 2014 by George McCollister Committed by Ingo Molnar Feb 27, 2014

sched: Fix double normalization of vruntime



dequeue_entity() is called when p->on_rq and sets se->on_rq = 0
which appears to guarentee that the !se->on_rq condition is met.
If the task has done set_current_state(TASK_INTERRUPTIBLE) without
schedule() the second condition will be met and vruntime will be
incorrectly adjusted twice.

In certain cases this can result in the task's vruntime never increasing
past the vruntime of other tasks on the CFS' run queue, starving them of
CPU time.

This patch changes switched_from_fair() to use !p->on_rq instead of
!se->on_rq.

I'm able to cause a task with a priority of 120 to starve all other
tasks with the same priority on an ARM platform running 3.2.51-rt72
PREEMPT RT by writing one character at time to a serial tty (16550 UART)
in a tight loop. I'm also able to verify making this change corrects the
problem on that platform and kernel version.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1392767811-28916-1-git-send-email-george.mccollister@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent d2a04763

kernel/sched/fair.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -7001,15 +7001,15 @@ static void switched_from_fair(struct rq rq, struct task_struct p)
		struct cfs_rq *cfs_rq = cfs_rq_of(se);

		/*
		* Ensure the task's vruntime is normalized, so that when its
		* Ensure the task's vruntime is normalized, so that when it's
		* switched back to the fair class the enqueue_entity(.flags=0) will
		* do the right thing.
		*
		* If it was on_rq, then the dequeue_entity(.flags=0) will already
		* have normalized the vruntime, if it was !on_rq, then only when
		* If it's on_rq, then the dequeue_entity(.flags=0) will already
		* have normalized the vruntime, if it's !on_rq, then only when
		* the task is sleeping will it still have non-normalized vruntime.
		*/
		if (!se->on_rq && p->state != TASK_RUNNING) {
		if (!p->on_rq && p->state != TASK_RUNNING) {
		/*
		* Fix up our vruntime so that the current sleep doesn't
		* cause 'unlimited' sleep bonus.

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