[v3] cpuidle: poll_state: Add time limit to poll_idle()

Commit Message

Rafael J. Wysocki March 14, 2018, 2:08 p.m. UTC

From: Rafael J. Wysocki <[e-mail]>

If poll_idle() is allowed to spin until need_resched() returns 'true',
it may actually spin for a much longer time than expected by the idle
governor, since set_tsk_need_resched() is not always called by the
timer interrupt handler.  If that happens, the CPU may spend much
more time than anticipated in the "polling" state.

To prevent that from happening, limit the time of the spinning loop
in poll_idle().

Suggested-by: Peter Zijlstra <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>
---

v2 -> v3: Use local_clock() for time measurements and drop the
          counter, since that should be lightweight enough (as
          suggested by Peter).

---
 drivers/cpuidle/poll_state.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

Comments

Rik van Riel March 22, 2018, 4:32 p.m. UTC | #1

On Wed, 2018-03-14 at 15:08 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <[e-mail]>
> 
> If poll_idle() is allowed to spin until need_resched() returns
> 'true',
> it may actually spin for a much longer time than expected by the idle
> governor, since set_tsk_need_resched() is not always called by the
> timer interrupt handler.  If that happens, the CPU may spend much
> more time than anticipated in the "polling" state.
> 
> To prevent that from happening, limit the time of the spinning loop
> in poll_idle().
> 
> Suggested-by: Peter Zijlstra <[e-mail]>
> Signed-off-by: Rafael J. Wysocki <[e-mail]>

So ... about bisecting that other patch series...

It turned out I had this patch, which looks so
obviously correct, as patch #1 in my series.

It also turned out that this patch is responsible
for the entire 5-10% increase in CPU use for the
memcache style workload.

I wonder if keeping an idle HT thread much busier
than before slows down its sibling, or something
like that.

Let me go test the nohz idle series by itself,
without this patch.

Patch

Index: linux-pm/drivers/cpuidle/poll_state.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/poll_state.c
+++ linux-pm/drivers/cpuidle/poll_state.c
@@ -6,15 +6,24 @@ 
 
 #include <linux/cpuidle.h>
 #include <linux/sched.h>
+#include <linux/sched/clock.h>
 #include <linux/sched/idle.h>
 
+#define POLL_IDLE_TIME_LIMIT	(TICK_NSEC / 16)
+
 static int __cpuidle poll_idle(struct cpuidle_device *dev,
 			       struct cpuidle_driver *drv, int index)
 {
+	u64 time_start = local_clock();
+
 	local_irq_enable();
 	if (!current_set_polling_and_test()) {
-		while (!need_resched())
+		while (!need_resched()) {
 			cpu_relax();
+
+			if (local_clock() - time_start > POLL_IDLE_TIME_LIMIT)
+				break;
+		}
 	}
 	current_clr_polling();