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Commit 4036ac15 authored by Mike Galbraith's avatar Mike Galbraith Committed by Ingo Molnar
Browse files

sched: Fix clock_gettime(CLOCK_[PROCESS/THREAD]_CPUTIME_ID) monotonicity 



If a task has been dequeued, it has been accounted.  Do not project
cycles that may or may not ever be accounted to a dequeued task, as
that may make clock_gettime() both inaccurate and non-monotonic.

Protect update_rq_clock() from slight TSC skew while at it.

Signed-off-by: default avatarMike Galbraith <umgwanakikbuti@gmail.com>
Cc: kosaki.motohiro@jp.fujitsu.com
Cc: pjt@google.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1403588980.29711.11.camel@marge.simpson.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent c06f04c7

kernel/sched/core.c

+11 −2
Original line number Diff line number Diff line
@@ -139,6 +139,8 @@ void update_rq_clock(struct rq *rq) 
		return;



	delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;

	if (delta < 0)

		return;

	rq->clock += delta;

	update_rq_clock_task(rq, delta);

}
@@ -2431,7 +2433,12 @@ static u64 do_task_delta_exec(struct task_struct *p, struct rq *rq) 
{

	u64 ns = 0;



	if (task_current(rq, p)) {

	/*

	 * Must be ->curr _and_ ->on_rq.  If dequeued, we would

	 * project cycles that may never be accounted to this

	 * thread, breaking clock_gettime().

	 */

	if (task_current(rq, p) && p->on_rq) {

		update_rq_clock(rq);

		ns = rq_clock_task(rq) - p->se.exec_start;

		if ((s64)ns < 0)
@@ -2474,8 +2481,10 @@ unsigned long long task_sched_runtime(struct task_struct *p) 
	 * If we race with it leaving cpu, we'll take a lock. So we're correct.

	 * If we race with it entering cpu, unaccounted time is 0. This is

	 * indistinguishable from the read occurring a few cycles earlier.

	 * If we see ->on_cpu without ->on_rq, the task is leaving, and has

	 * been accounted, so we're correct here as well.

	 */

	if (!p->on_cpu)

	if (!p->on_cpu || !p->on_rq)

		return p->se.sum_exec_runtime;

#endif