linux-kernel - Re: [PATCH] idle/tick-broadcast: Exit cpu idle poll loop when cleared from tick_broadcast_force

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Date:	Wed, 21 Jan 2015 10:56:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
cc:	peterz@...radead.org, linuxppc-dev@...ts.ozlabs.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] idle/tick-broadcast: Exit cpu idle poll loop when cleared
 from tick_broadcast_force_mask

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 <preeti@...ux.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?

Thanks,

	tglx
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