When a task exits, it notifies the parent that it has exited. This is a sync wakup and the exiting task may pull the parent towards the wakers CPU. For even simple workloads like using a shell, it was observed that the shell is pulled across nodes by exiting processes. This is daft as the parent may be long-lived and properly placed. This patch special cases a sync wakeup on exit to avoid pulling tasks across nodes. Testing on a range of workloads and machines showed very little differences in performance although there was a small 3% boost on some machines running a shellscript intensive workload (git regression test suite).
Signed-off-by: Mel Gorman <[email protected]> --- kernel/sched/fair.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 28c8d9c91955..50442697b455 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5710,8 +5710,14 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync) if (idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu)) return idle_cpu(prev_cpu) ? prev_cpu : this_cpu; - if (sync && cpu_rq(this_cpu)->nr_running == 1) + if (sync && cpu_rq(this_cpu)->nr_running == 1) { + /* Avoid tasks exiting pulling parents to new nodes */ + if ((current->flags & PF_EXITING) && + !cpus_share_cache(this_cpu, prev_cpu)) + return prev_cpu; + return this_cpu; + } return nr_cpumask_bits; } -- 2.15.1
