linux-kernel - Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll

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Date:   Thu, 22 Mar 2018 18:09:23 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Rik van Riel <riel@...riel.com>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll_idle()

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@...el.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@...radead.org>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.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;
 		}