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

Rafael J. Wysocki
2018-03-22T17:09:01Z

On Thursday, March 22, 2018 5:32:23 PM CET Rik van Riel wrote:
> On Wed, 2018-03-14 at 15:08 +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >
> > 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 <peterz@infradead.org>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> 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.

Uhm, sorry about this.

Does it improve if you do something like the below on top of it?

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

OK

---
drivers/cpuidle/poll_state.c | 6 ++++++
1 file changed, 6 insertions(+)

Index: linux-pm/drivers/cpuidle/poll_state.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/poll_state.c
+++ linux-pm/drivers/cpuidle/poll_state.c
@@ -10,6 +10,7 @@
#include <linux/sched/idle.h>

#define POLL_IDLE_TIME_LIMIT (TICK_NSEC / 16)
+#define POLL_IDLE_COUNT 1000

static int __cpuidle poll_idle(struct cpuidle_device *dev,
struct cpuidle_driver *drv, int index)
@@ -18,9 +19,14 @@ static int __cpuidle poll_idle(struct cp

local_irq_enable();
if (!current_set_polling_and_test()) {
+ unsigned int loop_count = 0;
+
while (!need_resched()) {
cpu_relax();
+ if (loop_count++ < POLL_IDLE_COUNT)
+ continue;

+ loop_count = 0;
if (local_clock() - time_start > POLL_IDLE_TIME_LIMIT)
break;
}