[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 <redacted>
---

 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();

On Mon, 19 Jan 2015, Preeti U Murthy wrote:

You explain the tick_check_broadcast_expired() bit, but what about the
cpu_idle_force_poll part?

Thanks,

	tglx

On 01/20/2015 04:51 PM, Thomas Gleixner wrote:

On Mon, 19 Jan 2015, Preeti U Murthy wrote:

quoted

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 <redacted>
---

 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.
Perhaps I should s/poll explicitly/do cpu_idle_force_poll ?

Regards
Preeti U Murthy

Thanks,

	tglx

On Tue, 20 Jan 2015, Preeti U Murthy wrote:

On 01/20/2015 04:51 PM, Thomas Gleixner wrote:

quoted

On Mon, 19 Jan 2015, Preeti U Murthy wrote:

quoted

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 <redacted>
---

 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

On 01/21/2015 03:26 PM, Thomas Gleixner wrote:

On Tue, 20 Jan 2015, Preeti U Murthy wrote:

quoted

On 01/20/2015 04:51 PM, Thomas Gleixner wrote:

quoted

On Mon, 19 Jan 2015, Preeti U Murthy wrote:

quoted

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 <redacted>
---

 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