Commit a860fa7b authored Apr 20, 2019 by Xie XiuQi Committed by Ingo Molnar Apr 25, 2019

sched/numa: Fix a possible divide-by-zero



sched_clock_cpu() may not be consistent between CPUs. If a task
migrates to another CPU, then se.exec_start is set to that CPU's
rq_clock_task() by update_stats_curr_start(). Specifically, the new
value might be before the old value due to clock skew.

So then if in numa_get_avg_runtime() the expression:

  'now - p->last_task_numa_placement'

ends up as -1, then the divider '*period + 1' in task_numa_placement()
is 0 and things go bang. Similar to update_curr(), check if time goes
backwards to avoid this.

[ peterz: Wrote new changelog. ]
[ mingo: Tweaked the code comment. ]

Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: cj.chengjian@huawei.com
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20190425080016.GX11158@hirez.programming.kicks-ass.net


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

parent cd8dead0

kernel/sched/fair.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -2007,6 +2007,10 @@ static u64 numa_get_avg_runtime(struct task_struct p, u64 period)
		if (p->last_task_numa_placement) {
		delta = runtime - p->last_sum_exec_runtime;
		*period = now - p->last_task_numa_placement;

		/* Avoid time going backwards, prevent potential divide error: */
		if (unlikely((s64)*period < 0))
		*period = 0;
		} else {
		delta = p->se.avg.load_sum;
		*period = LOAD_AVG_MAX;

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