On Tue, Jun 04, 2013 at 01:15:10PM +0200, Peter Zijlstra wrote:
On Tue, Jun 04, 2013 at 12:26:22PM +0200, Frederic Weisbecker wrote:
@@ -1393,8 +1392,12 @@ static void sched_ttwu_pending(void) void scheduler_ipi(void) {
- if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick()
&& !tick_nohz_full_cpu(smp_processor_id()))
- int cpu = smp_processor_id();
- bool idle_kick = got_nohz_idle_kick(cpu);
This puts an unconditional atomic instruction in the IPI path. if (test) clear(); is lots cheaper, esp. since most IPIs won't have this flag set.
Agreed but I'm a bit worried about ordering:
CPU 0 CPU 1
test_and_set_bit(nohz_kick, CPU 1) scheduler_ipi smp_send_reschedule(CPU 1) if (test_and_clear_bit(nohz_kick)) do_something
I'm not sure what base guarantee we have with ordering against raw IPIs such as the the scheduler ipi. But unless both IPI trigger and IPI receive imply a full barrier (or just IPI receive implies read barrier, it seems that's all we need), we need test_and_set_bit() or smp_rmb()/smp_mb__before_clear_bit() && smp_mb__after_clear_bit().
- if (!(idle_kick && idle_cpu(cpu))
&& llist_empty(&this_rq()->wake_list)&& !tick_nohz_full_cpu(cpu)What's with this weird operator first split style?
Yeah ugly, I'll fix.
return;/*
+enum idle_balance_type {
- IDLE_BALANCE = 1,
- IDLE_NOHZ_BALANCE = 2,
+};
You might want to update the rq->idle_balance assignment in scheduler_tick() to make sure it uses the right value (it does now, but there's nothing stopping people from changing the values).
Agreed!
Thanks.