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Re: [PATCH RFC] select_idle_sibling experiments

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  Re: [PATCH RFC] select_idle_sibling experiments Peter Zijlstra <peterz@infradead.org> - 2016-04-28 14:10 +0200
    Re: [PATCH RFC] select_idle_sibling experiments Mike Galbraith <mgalbraith@suse.de> - 2016-04-28 15:20 +0200
    Re: [PATCH RFC] select_idle_sibling experiments Mike Galbraith <mgalbraith@suse.de> - 2016-05-02 07:40 +0200

#1390102 — Re: [PATCH RFC] select_idle_sibling experiments

On Wed, Apr 06, 2016 at 09:27:24AM +0200, Mike Galbraith wrote:
> sched: ratelimit nohz
> 
> Entering nohz code on every micro-idle is too expensive to bear.
> 
> Signed-off-by: Mike Galbraith <efault@gmx.de>

> +int sched_needs_cpu(int cpu)
> +{
> +	if (tick_nohz_full_cpu(cpu))
> +		return 0;
> +
> +	return  cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost;

So the only problem I have with this patch is the choice of limit. This
isn't at all tied to the migration cost.

And some people are already twiddling with the migration_cost knob to
affect the idle_balance() behaviour -- making it much more agressive by
dialing it down. When you do that you also loose the effectiveness of
this proposed usage, even though those same people would probably want
this.

Failing a spot of inspiration for a runtime limit on this; we might have
to introduce yet another knob :/

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#1390155

On Thu, 2016-04-28 at 14:00 +0200, Peter Zijlstra wrote:
> On Wed, Apr 06, 2016 at 09:27:24AM +0200, Mike Galbraith wrote:
> > sched: ratelimit nohz
> > 
> > Entering nohz code on every micro-idle is too expensive to bear.
> > 
> > Signed-off-by: Mike Galbraith <efault@gmx.de>
> 
> > +int sched_needs_cpu(int cpu)
> > +{
> > +> > 	> > if (tick_nohz_full_cpu(cpu))
> > +> > 	> > 	> > return 0;
> > +
> > +> > 	> > return  cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost;
> 
> So the only problem I have with this patch is the choice of limit. This
> isn't at all tied to the migration cost.

Yup.

> And some people are already twiddling with the migration_cost knob to
> affect the idle_balance() behaviour -- making it much more agressive by
> dialing it down. When you do that you also loose the effectiveness of
> this proposed usage, even though those same people would probably want
> this.
> 
> Failing a spot of inspiration for a runtime limit on this; we might have
> to introduce yet another knob :/

I'll roll one with a yet another of it's very own.

	-Mike

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#1391988

On Thu, 2016-04-28 at 14:00 +0200, Peter Zijlstra wrote:
> On Wed, Apr 06, 2016 at 09:27:24AM +0200, Mike Galbraith wrote:
> > sched: ratelimit nohz
> > 
> > Entering nohz code on every micro-idle is too expensive to bear.
> > 
> > Signed-off-by: Mike Galbraith <efault@gmx.de>
> 
> > +int sched_needs_cpu(int cpu)
> > +{
> > +> > 	> > if (tick_nohz_full_cpu(cpu))
> > +> > 	> > 	> > return 0;
> > +
> > +> > 	> > return  cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost;
> 
> So the only problem I have with this patch is the choice of limit. This
> isn't at all tied to the migration cost.
> 
> And some people are already twiddling with the migration_cost knob to
> affect the idle_balance() behaviour -- making it much more agressive by
> dialing it down. When you do that you also loose the effectiveness of
> this proposed usage, even though those same people would probably want
> this.
> 
> Failing a spot of inspiration for a runtime limit on this; we might have
> to introduce yet another knob :/

sched: ratelimit nohz tick shutdown/restart

Tick shutdown/restart overhead can be substantial when CPUs
enter/exit the idle loop at high frequency.  Ratelimit based
upon rq->avg_idle, and provide an adjustment knob.

Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 include/linux/sched.h        |    5 +++++
 include/linux/sched/sysctl.h |    4 ++++
 kernel/sched/core.c          |   10 ++++++++++
 kernel/sysctl.c              |    9 +++++++++
 kernel/time/tick-sched.c     |    2 +-
 5 files changed, 29 insertions(+), 1 deletion(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2286,6 +2286,11 @@ static inline int set_cpus_allowed_ptr(s
 #ifdef CONFIG_NO_HZ_COMMON
 void calc_load_enter_idle(void);
 void calc_load_exit_idle(void);
+#ifdef CONFIG_SMP
+extern int sched_needs_cpu(int cpu);
+#else
+static inline int sched_needs_cpu(int cpu) { return 0; }
+#endif
 #else
 static inline void calc_load_enter_idle(void) { }
 static inline void calc_load_exit_idle(void) { }
--- a/include/linux/sched/sysctl.h
+++ b/include/linux/sched/sysctl.h
@@ -19,6 +19,10 @@ extern unsigned int sysctl_sched_min_gra
 extern unsigned int sysctl_sched_wakeup_granularity;
 extern unsigned int sysctl_sched_child_runs_first;
 
+#if defined(CONFIG_NO_HZ_COMMON) && defined(CONFIG_SMP)
+extern unsigned int sysctl_sched_nohz_throttle;
+#endif
+
 enum sched_tunable_scaling {
 	SCHED_TUNABLESCALING_NONE,
 	SCHED_TUNABLESCALING_LOG,
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -577,6 +577,16 @@ static inline bool got_nohz_idle_kick(vo
 	return false;
 }
 
+unsigned int sysctl_sched_nohz_throttle = 500000UL;
+
+int sched_needs_cpu(int cpu)
+{
+	if (tick_nohz_full_cpu(cpu))
+		return 0;
+
+	return  cpu_rq(cpu)->avg_idle < sysctl_sched_nohz_throttle;
+}
+
 #else /* CONFIG_NO_HZ_COMMON */
 
 static inline bool got_nohz_idle_kick(void)
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -351,6 +351,15 @@ static struct ctl_table kern_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+#ifdef CONFIG_NO_HZ_COMMON
+	{
+		.procname	= "sched_nohz_throttle_ns",
+		.data		= &sysctl_sched_nohz_throttle,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
+#endif
 #ifdef CONFIG_SCHEDSTATS
 	{
 		.procname	= "sched_schedstats",
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -676,7 +676,7 @@ static ktime_t tick_nohz_stop_sched_tick
 	} while (read_seqretry(&jiffies_lock, seq));
 	ts->last_jiffies = basejiff;
 
-	if (rcu_needs_cpu(basemono, &next_rcu) ||
+	if (sched_needs_cpu(cpu) || rcu_needs_cpu(basemono, &next_rcu) ||
 	    arch_needs_cpu() || irq_work_needs_cpu()) {
 		next_tick = basemono + TICK_NSEC;
 	} else {

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