On 01/21/2015 03:26 PM, Thomas Gleixner wrote: > On Tue, 20 Jan 2015, Preeti U Murthy wrote: >> On 01/20/2015 04:51 PM, Thomas Gleixner wrote: >>> On Mon, 19 Jan 2015, Preeti U Murthy wrote: >>>> An idle cpu enters cpu_idle_poll() if it is set in the >>>> tick_broadcast_force_mask. >>>> This is so that it does not incur the overhead of entering idle states >>>> when it is expected >>>> to be woken up anytime then through a broadcast IPI. The condition that >>>> forces an exit out >>>> of the idle polling is the check on setting of the TIF_NEED_RESCHED flag >>>> for the idle thread. >>>> >>>> When the broadcast IPI does arrive, it is not guarenteed that the handler >>>> sets the >>>> TIF_NEED_RESCHED flag. Hence although the cpu is cleared in the >>>> tick_broadcast_force_mask, >>>> it continues to loop in cpu_idle_poll unnecessarily wasting power. Hence >>>> exit the idle >>>> poll loop if the tick_broadcast_force_mask gets cleared and enter idle >>>> states. >>>> >>>> Of course if the cpu has entered cpu_idle_poll() on being asked to poll >>>> explicitly, >>>> it continues to poll till it is asked to reschedule. >>>> >>>> Signed-off-by: Preeti U Murthy <pre...@linux.vnet.ibm.com> >>>> --- >>>> >>>> kernel/sched/idle.c | 3 ++- >>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c >>>> index c47fce7..aaf1c1d 100644 >>>> --- a/kernel/sched/idle.c >>>> +++ b/kernel/sched/idle.c >>>> @@ -47,7 +47,8 @@ static inline int cpu_idle_poll(void) >>>> rcu_idle_enter(); >>>> trace_cpu_idle_rcuidle(0, smp_processor_id()); >>>> local_irq_enable(); >>>> - while (!tif_need_resched()) >>>> + while (!tif_need_resched() && >>>> + (cpu_idle_force_poll || tick_check_broadcast_expired())) >>> >>> You explain the tick_check_broadcast_expired() bit, but what about the >>> cpu_idle_force_poll part? >> >> The last few lines which say "Of course if the cpu has entered >> cpu_idle_poll() on being asked to poll explicitly, it continues to poll >> till it is asked to reschedule" explains the cpu_idle_force_poll part. > > Well, I read it more than once and did not figure it out. > > The paragraph describes some behaviour. Now I know it's the behaviour > before the patch. So maybe something like this: > > cpu_idle_poll() is entered when cpu_idle_force_poll is set or > tick_check_broadcast_expired() returns true. The exit condition from > cpu_idle_poll() is tif_need_resched(). > > But this does not take into account that cpu_idle_force_poll and > tick_check_broadcast_expired() can change without setting the > resched flag. So a cpu can be caught in cpu_idle_poll() needlessly > and thereby wasting power. > > Add an explicit check for cpu_idle_force_poll and > tick_check_broadcast_expired() to the exit condition of > cpu_idle_poll() to avoid this. > > This explains the technical issue without confusing people with IPIs > and other completely irrelevant information. Hmm?
Yep, much simpler, thanks! I will send out the next version with this changelog. Regards Preeti U Murthy > > Thanks, > > tglx > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev > _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
