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

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Date:	Mon, 19 Jan 2015 10:57:54 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	tglx@...utronix.de
Cc:	peterz@...radead.org, linuxppc-dev@...ts.ozlabs.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] idle/tick-broadcast: Exit cpu idle poll loop when cleared
 from tick_broadcast_force_mask

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()))
 		cpu_relax();
 	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
 	rcu_idle_exit();

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