sched/numa: Fix math underflow in task_tick_numa()

The NUMA balancing code implements delays in scanning by
advancing curr->node_stamp beyond curr->se.sum_exec_runtime.

With unsigned math, that creates an underflow, which results
in task_numa_work being queued all the time, even when we
don't want to.

Avoiding the math underflow makes it possible to reduce CPU
overhead in the NUMA balancing code.

Reported-and-tested-by: Jan Stancek <[email protected]>
Signed-off-by: Rik van Riel <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 824aa9f501a3b2a183a5a700dee57c984c51f89e..f04fda8f669c8eff57e936fcb1334b13a4140ce5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2302,7 +2302,7 @@ void task_tick_numa(struct rq *rq, struct task_struct *curr)
        now = curr->se.sum_exec_runtime;
        period = (u64)curr->numa_scan_period * NSEC_PER_MSEC;
 
-       if (now - curr->node_stamp > period) {
+       if (now > curr->node_stamp + period) {
                if (!curr->node_stamp)
                        curr->numa_scan_period = task_scan_min(curr);
                curr->node_stamp += period;