On 23 January 2014 19:05, Frederic Weisbecker <[email protected]> wrote: > On Thu, Jan 23, 2014 at 11:22:32AM +0530, Viresh Kumar wrote: >> I think below diff might get this fixed for you, though I am not sure if it >> breaks something else. Probably Thomas/Frederic can answer here. >> If this looks fine I will send it formally again: >> >> diff --git a/kernel/timer.c b/kernel/timer.c >> index accfd24..3a2c7fa 100644 >> --- a/kernel/timer.c >> +++ b/kernel/timer.c >> @@ -940,7 +940,8 @@ void add_timer_on(struct timer_list *timer, int cpu) >> * makes sure that a CPU on the way to stop its tick can not >> * evaluate the timer wheel. >> */ >> - wake_up_nohz_cpu(cpu); >> + if (!tbase_get_deferrable(timer->base)) >> + wake_up_nohz_cpu(cpu);
Wait, I got the wrong code here. That's wasn't my initial intention. I actually wanted to write something like this: - wake_up_nohz_cpu(cpu); + if (!tbase_get_deferrable(timer->base) || idle_cpu(cpu)) + wake_up_nohz_cpu(cpu); Will that work? > So you simply rely on the next tick to see the new timer. This should work > with > CONFIG_NO_HZ_IDLE but not with CONFIG_NO_HZ_FULL since the target may be > running > without the tick. > > Basically, in the case of a deferrable timer you need to manage to call > wake_up_full_nohz_cpu() but not wake_up_idle_cpu(). > > It should be even possible to spare the IPI in a full dynticks CPU if it is > running idle. But that's an optional bonus because it require some deep > care on complicated races against the call to tick_nohz_idle_exit(). > > I also realize than when we enqueue a timer on a full nohz CPU, we should > set_need_resched() > the target before sending the IPI if it is idle like does wake_up_idle_cpu(). > Otherwise the > IPI will be ignored without exiting the idle loop nor reevaluating the tick > on irq exit. > If you can fix that along the way, that will be much appreciated. I haven't thought much about this currently as I have limited knowledge of these routines. Though the problem we were facing wasn't related to NO_HZ_FULL. It was just about waking up an idle cpu. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
