[PATCH v5 0/2] sched: consider missed ticks when updating cpu load

[PATCH v5 0/2] sched: consider missed ticks when updating cpu load

[byungchul.park]

From: Byungchul Park <byungchul.park@lge.com>

change from v4 to v5
- fix comments and commit message
- take new_load into account in update_cpu_load_nohz() and __update_cpu_load()
  because it's non-zero in NOHZ_FULL

change from v3 to v4
- focus the problem on full NOHZ

change from v2 to v3
- add a patch which make __update_cpu_load() handle active tickless

change from v1 to v2
- add some additional commit message (logic is same exactly)

additionally i tried to use the return value of hrtimer_forward() instead of
jiffies to get pending tick for updating cpu load. it's easy for
update_cpu_load_nohz() to do that. but for update_idle_cpu_load(), i need
more time to think about how to implement it nicely.

and i will try to fix other stuffs caused by full NOHZ later with another
patch. i decided to send this patch at first because "cpu load update" is a
standalone problem which is not coupled with other stuffs.

Byungchul Park (2):
  sched: make __update_cpu_load() handle NOHZ_FULL tickless
  sched: make update_cpu_load_nohz() consider missed ticks in NOHZ_FULL

 include/linux/sched.h    |    4 ++--
 kernel/sched/fair.c      |   57 +++++++++++++++++++++++++++++++++++++---------
 kernel/time/tick-sched.c |    8 +++----
 3 files changed, 52 insertions(+), 17 deletions(-)

-- 
1.7.9.5

[PATCH v5 1/2] sched: make __update_cpu_load() handle NOHZ_FULL tickless

[byungchul.park]

From: Byungchul Park <byungchul.park@lge.com>

__update_cpu_load() assumes that a cpu is idle if the interval between
the cpu's ticks is more than 1/HZ. However the cpu can be non-idle even
though the interval is more than 1/HZ, in the NOHZ_FULL case.

Thus in the NOHZ_FULL tickless case, the current way to update cpu load
can be wrong. To update cpu load properly, __update_cpu_load() should be
changed so that it can handle NOHZ_FULL tickless case.

A decaying average can be computed by

  load[i]' = (1 - 1/2^i) * load[i] + (1/2^i) * load

Because of NOHZ, it might not get called on every tick which gives need
for the @pending_updates argument.

  load[i]_n = (1 - 1/2^i) * load[i]_n-1 + (1/2^i) * load_n-1
            = A * load[i]_n-1 + B ; A := (1 - 1/2^i), B := (1/2^i) * load
            = A * (A * load[i]_n-2 + B) + B
            = A * (A * (A * load[i]_n-3 + B) + B) + B
            = A^3 * load[i]_n-3 + (A^2 + A + 1) * B
            = A^n * load[i]_0 + (A^(n-1) + A^(n-2) + ... + 1) * B
            = A^n * load[i]_0 + ((1 - A^n) / (1 - A)) * B
            = (1 - 1/2^i)^n * (load[i]_0 - load) + load

In the above we've assumed load_n := load, which is true for NOHZ_FULL as
any change in load would have resulted in the tick being turned back on.

For regular NOHZ, this reduces to:

  load[i]_n = (1 - 1/2^i)^n * load[i]_0

[Commit message and comments become simpler, thanks to Peter Z.]

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
 kernel/sched/fair.c |   49 +++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 41 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 077076f..7760678 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4305,14 +4305,46 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
 	return load;
 }
 
-/*
+/**
+ * __update_cpu_load - update the rq->cpu_load[] statistics
+ * @this_rq: The rq to update statistics for
+ * @this_load: The current load
+ * @pending_updates: The number of missed updates
+ * @active: !0 for NOHZ_FULL
+ *
  * Update rq->cpu_load[] statistics. This function is usually called every
- * scheduler tick (TICK_NSEC). With tickless idle this will not be called
- * every tick. We fix it up based on jiffies.
+ * scheduler tick (TICK_NSEC).
+ *
+ * This function computes a decaying average:
+ *
+ *   load[i]' = (1 - 1/2^i) * load[i] + (1/2^i) * load
+ *
+ * Because of NOHZ it might not get called on every tick which gives need for
+ * the @pending_updates argument.
+ *
+ *   load[i]_n = (1 - 1/2^i) * load[i]_n-1 + (1/2^i) * load_n-1
+ *             = A * load[i]_n-1 + B ; A := (1 - 1/2^i), B := (1/2^i) * load
+ *             = A * (A * load[i]_n-2 + B) + B
+ *             = A * (A * (A * load[i]_n-3 + B) + B) + B
+ *             = A^3 * load[i]_n-3 + (A^2 + A + 1) * B
+ *             = A^n * load[i]_0 + (A^(n-1) + A^(n-2) + ... + 1) * B
+ *             = A^n * load[i]_0 + ((1 - A^n) / (1 - A)) * B
+ *             = (1 - 1/2^i)^n * (load[i]_0 - load) + load
+ *
+ * In the above we've assumed load_n := load, which is true for NOHZ_FULL as
+ * any change in load would have resulted in the tick being turned back on.
+ *
+ * For regular NOHZ, this reduces to:
+ *
+ *   load[i]_n = (1 - 1/2^i)^n * load[i]_0
+ *
+ * see decay_load_misses(). For NOHZ_FULL we get to subtract and add the extra
+ * term. See the @active paramter.
  */
 static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
-			      unsigned long pending_updates)
+			      unsigned long pending_updates, int active)
 {
+	unsigned long tickless_load = active ? this_rq->cpu_load[0] : 0;
 	int i, scale;
 
 	this_rq->nr_load_updates++;
@@ -4324,8 +4356,9 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
 
 		/* scale is effectively 1 << i now, and >> i divides by scale */
 
-		old_load = this_rq->cpu_load[i];
+		old_load = this_rq->cpu_load[i] - tickless_load;
 		old_load = decay_load_missed(old_load, pending_updates - 1, i);
+		old_load += tickless_load;
 		new_load = this_load;
 		/*
 		 * Round up the averaging division if load is increasing. This
@@ -4380,7 +4413,7 @@ static void update_idle_cpu_load(struct rq *this_rq)
 	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
 	this_rq->last_load_update_tick = curr_jiffies;
 
-	__update_cpu_load(this_rq, load, pending_updates);
+	__update_cpu_load(this_rq, load, pending_updates, 0);
 }
 
 /*
@@ -4403,7 +4436,7 @@ void update_cpu_load_nohz(void)
 		 * We were idle, this means load 0, the current load might be
 		 * !0 due to remote wakeups and the sort.
 		 */
-		__update_cpu_load(this_rq, 0, pending_updates);
+		__update_cpu_load(this_rq, 0, pending_updates, 0);
 	}
 	raw_spin_unlock(&this_rq->lock);
 }
@@ -4419,7 +4452,7 @@ void update_cpu_load_active(struct rq *this_rq)
 	 * See the mess around update_idle_cpu_load() / update_cpu_load_nohz().
 	 */
 	this_rq->last_load_update_tick = jiffies;
-	__update_cpu_load(this_rq, load, 1);
+	__update_cpu_load(this_rq, load, 1, 1);
 }
 
 /*
-- 
1.7.9.5

[PATCH v5 2/2] sched: make update_cpu_load_nohz() consider missed ticks in NOHZ_FULL

[byungchul.park]

From: Byungchul Park <byungchul.park@lge.com>

Usually tick can be stopped for an idle cpu in NOHZ. However in NOHZ_FULL,
a non-idle cpu's tick also can be stopped. However, update_cpu_load_nohz()
does not consider the case a non-idle cpu's tick has been stopped at all.

This patch makes the update_cpu_load_nohz() know if the calling path comes
from NOHZ_FULL or idle NOHZ.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 include/linux/sched.h    |    4 ++--
 kernel/sched/fair.c      |   10 ++++++----
 kernel/time/tick-sched.c |    8 ++++----
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 699228b..f1b59aa 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -177,9 +177,9 @@ extern void get_iowait_load(unsigned long *nr_waiters, unsigned long *load);
 extern void calc_global_load(unsigned long ticks);
 
 #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
-extern void update_cpu_load_nohz(void);
+extern void update_cpu_load_nohz(int active);
 #else
-static inline void update_cpu_load_nohz(void) { }
+static inline void update_cpu_load_nohz(int active) { }
 #endif
 
 extern unsigned long get_parent_ip(unsigned long addr);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7760678..3d93756 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4419,10 +4419,11 @@ static void update_idle_cpu_load(struct rq *this_rq)
 /*
  * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
  */
-void update_cpu_load_nohz(void)
+void update_cpu_load_nohz(int active)
 {
 	struct rq *this_rq = this_rq();
 	unsigned long curr_jiffies = READ_ONCE(jiffies);
+	unsigned long load = active ? weighted_cpuload(cpu_of(this_rq)) : 0;
 	unsigned long pending_updates;
 
 	if (curr_jiffies == this_rq->last_load_update_tick)
@@ -4433,10 +4434,11 @@ void update_cpu_load_nohz(void)
 	if (pending_updates) {
 		this_rq->last_load_update_tick = curr_jiffies;
 		/*
-		 * We were idle, this means load 0, the current load might be
-		 * !0 due to remote wakeups and the sort.
+		 * In the regular NOHZ case, we were idle, this means load 0.
+		 * In the NOHZ_FULL case, we were non-idle, we should consider
+		 * its weighted load.
 		 */
-		__update_cpu_load(this_rq, 0, pending_updates, 0);
+		__update_cpu_load(this_rq, load, pending_updates, active);
 	}
 	raw_spin_unlock(&this_rq->lock);
 }
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7c7ec45..515edf3 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -694,11 +694,11 @@ out:
 	return tick;
 }
 
-static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
+static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now, int active)
 {
 	/* Update jiffies first */
 	tick_do_update_jiffies64(now);
-	update_cpu_load_nohz();
+	update_cpu_load_nohz(active);
 
 	calc_load_exit_idle();
 	touch_softlockup_watchdog();
@@ -725,7 +725,7 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
 	if (can_stop_full_tick())
 		tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
 	else if (ts->tick_stopped)
-		tick_nohz_restart_sched_tick(ts, ktime_get());
+		tick_nohz_restart_sched_tick(ts, ktime_get(), 1);
 #endif
 }
 
@@ -916,7 +916,7 @@ void tick_nohz_idle_exit(void)
 		tick_nohz_stop_idle(ts, now);
 
 	if (ts->tick_stopped) {
-		tick_nohz_restart_sched_tick(ts, now);
+		tick_nohz_restart_sched_tick(ts, now, 0);
 		tick_nohz_account_idle_ticks(ts);
 	}
 
-- 
1.7.9.5

Re: [PATCH v5 0/2] sched: consider missed ticks when updating cpu load

[Byungchul Park]

On Tue, Nov 10, 2015 at 09:36:00AM +0900, byungchul.park@lge.com wrote:
> From: Byungchul Park <byungchul.park@lge.com>
> 
> change from v4 to v5
> - fix comments and commit message
> - take new_load into account in update_cpu_load_nohz() and __update_cpu_load()
>   because it's non-zero in NOHZ_FULL
> 
> change from v3 to v4
> - focus the problem on full NOHZ
> 
> change from v2 to v3
> - add a patch which make __update_cpu_load() handle active tickless
> 
> change from v1 to v2
> - add some additional commit message (logic is same exactly)
> 
> additionally i tried to use the return value of hrtimer_forward() instead of
> jiffies to get pending tick for updating cpu load. it's easy for
> update_cpu_load_nohz() to do that. but for update_idle_cpu_load(), i need
> more time to think about how to implement it nicely.

Actually because of update_idle_load() which is used when doing nohz
load balancing while the cpu has been still tick-stopped, this looks
not be able to be implemented without any kind of last_update_xxx
variable. What do you think about it, Peter?

> 
> and i will try to fix other stuffs caused by full NOHZ later with another
> patch. i decided to send this patch at first because "cpu load update" is a
> standalone problem which is not coupled with other stuffs.
> 
> Byungchul Park (2):
>   sched: make __update_cpu_load() handle NOHZ_FULL tickless
>   sched: make update_cpu_load_nohz() consider missed ticks in NOHZ_FULL
> 
>  include/linux/sched.h    |    4 ++--
>  kernel/sched/fair.c      |   57 +++++++++++++++++++++++++++++++++++++---------
>  kernel/time/tick-sched.c |    8 +++----
>  3 files changed, 52 insertions(+), 17 deletions(-)
> 
> -- 
> 1.7.9.5
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v5 2/2] sched: make update_cpu_load_nohz() consider missed ticks in NOHZ_FULL

[Byungchul Park]

On Tue, Nov 10, 2015 at 09:36:02AM +0900, byungchul.park@lge.com wrote:
> From: Byungchul Park <byungchul.park@lge.com>
> 
> Usually tick can be stopped for an idle cpu in NOHZ. However in NOHZ_FULL,
> a non-idle cpu's tick also can be stopped. However, update_cpu_load_nohz()
> does not consider the case a non-idle cpu's tick has been stopped at all.
> 
> This patch makes the update_cpu_load_nohz() know if the calling path comes
> from NOHZ_FULL or idle NOHZ.
> 
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> ---
>  include/linux/sched.h    |    4 ++--
>  kernel/sched/fair.c      |   10 ++++++----
>  kernel/time/tick-sched.c |    8 ++++----
>  3 files changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 699228b..f1b59aa 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -177,9 +177,9 @@ extern void get_iowait_load(unsigned long *nr_waiters, unsigned long *load);
>  extern void calc_global_load(unsigned long ticks);
>  
>  #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
> -extern void update_cpu_load_nohz(void);
> +extern void update_cpu_load_nohz(int active);
>  #else
> -static inline void update_cpu_load_nohz(void) { }
> +static inline void update_cpu_load_nohz(int active) { }
>  #endif
>  
>  extern unsigned long get_parent_ip(unsigned long addr);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7760678..3d93756 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4419,10 +4419,11 @@ static void update_idle_cpu_load(struct rq *this_rq)
>  /*
>   * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
>   */
> -void update_cpu_load_nohz(void)
> +void update_cpu_load_nohz(int active)
>  {
>  	struct rq *this_rq = this_rq();
>  	unsigned long curr_jiffies = READ_ONCE(jiffies);
> +	unsigned long load = active ? weighted_cpuload(cpu_of(this_rq)) : 0;

Hello,

I added this part after you acked.. What do you think about this
modification?

>  	unsigned long pending_updates;
>  
>  	if (curr_jiffies == this_rq->last_load_update_tick)
> @@ -4433,10 +4434,11 @@ void update_cpu_load_nohz(void)
>  	if (pending_updates) {
>  		this_rq->last_load_update_tick = curr_jiffies;
>  		/*
> -		 * We were idle, this means load 0, the current load might be
> -		 * !0 due to remote wakeups and the sort.
> +		 * In the regular NOHZ case, we were idle, this means load 0.
> +		 * In the NOHZ_FULL case, we were non-idle, we should consider
> +		 * its weighted load.

This, too.

>  		 */
> -		__update_cpu_load(this_rq, 0, pending_updates, 0);
> +		__update_cpu_load(this_rq, load, pending_updates, active);

This, too.

Thank you,
byungchul

>  	}
>  	raw_spin_unlock(&this_rq->lock);
>  }
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 7c7ec45..515edf3 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -694,11 +694,11 @@ out:
>  	return tick;
>  }
>  
> -static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
> +static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now, int active)
>  {
>  	/* Update jiffies first */
>  	tick_do_update_jiffies64(now);
> -	update_cpu_load_nohz();
> +	update_cpu_load_nohz(active);
>  
>  	calc_load_exit_idle();
>  	touch_softlockup_watchdog();
> @@ -725,7 +725,7 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
>  	if (can_stop_full_tick())
>  		tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
>  	else if (ts->tick_stopped)
> -		tick_nohz_restart_sched_tick(ts, ktime_get());
> +		tick_nohz_restart_sched_tick(ts, ktime_get(), 1);
>  #endif
>  }
>  
> @@ -916,7 +916,7 @@ void tick_nohz_idle_exit(void)
>  		tick_nohz_stop_idle(ts, now);
>  
>  	if (ts->tick_stopped) {
> -		tick_nohz_restart_sched_tick(ts, now);
> +		tick_nohz_restart_sched_tick(ts, now, 0);
>  		tick_nohz_account_idle_ticks(ts);
>  	}
>  
> -- 
> 1.7.9.5
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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Re: [PATCH v5 0/2] sched: consider missed ticks when updating cpu load

[Byungchul Park]

On Wed, Nov 11, 2015 at 06:38:10PM +0900, Byungchul Park wrote:
> On Tue, Nov 10, 2015 at 09:36:00AM +0900, byungchul.park@lge.com wrote:
> > From: Byungchul Park <byungchul.park@lge.com>
> > 
> > change from v4 to v5
> > - fix comments and commit message
> > - take new_load into account in update_cpu_load_nohz() and __update_cpu_load()
> >   because it's non-zero in NOHZ_FULL
> > 
> > change from v3 to v4
> > - focus the problem on full NOHZ
> > 
> > change from v2 to v3
> > - add a patch which make __update_cpu_load() handle active tickless
> > 
> > change from v1 to v2
> > - add some additional commit message (logic is same exactly)
> > 
> > additionally i tried to use the return value of hrtimer_forward() instead of
> > jiffies to get pending tick for updating cpu load. it's easy for
> > update_cpu_load_nohz() to do that. but for update_idle_cpu_load(), i need
> > more time to think about how to implement it nicely.
> 
> Actually because of update_idle_load() which is used when doing nohz
> load balancing while the cpu has been still tick-stopped, this looks
> not be able to be implemented without any kind of last_update_xxx
                                                    ^^^
						    last_load_update_tick
> variable. What do you think about it, Peter?
> 
> > 
> > and i will try to fix other stuffs caused by full NOHZ later with another
> > patch. i decided to send this patch at first because "cpu load update" is a
> > standalone problem which is not coupled with other stuffs.
> > 
> > Byungchul Park (2):
> >   sched: make __update_cpu_load() handle NOHZ_FULL tickless
> >   sched: make update_cpu_load_nohz() consider missed ticks in NOHZ_FULL
> > 
> >  include/linux/sched.h    |    4 ++--
> >  kernel/sched/fair.c      |   57 +++++++++++++++++++++++++++++++++++++---------
> >  kernel/time/tick-sched.c |    8 +++----
> >  3 files changed, 52 insertions(+), 17 deletions(-)
> > 
> > -- 
> > 1.7.9.5
> > 
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v5 2/2] sched: make update_cpu_load_nohz() consider missed ticks in NOHZ_FULL

[Peter Zijlstra]

On Tue, Nov 10, 2015 at 09:36:02AM +0900, byungchul.park@lge.com wrote:
> +++ b/kernel/sched/fair.c
> @@ -4419,10 +4419,11 @@ static void update_idle_cpu_load(struct rq *this_rq)
>  /*
>   * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
>   */
> -void update_cpu_load_nohz(void)
> +void update_cpu_load_nohz(int active)
>  {
>  	struct rq *this_rq = this_rq();
>  	unsigned long curr_jiffies = READ_ONCE(jiffies);
> +	unsigned long load = active ? weighted_cpuload(cpu_of(this_rq)) : 0;
>  	unsigned long pending_updates;
>  
>  	if (curr_jiffies == this_rq->last_load_update_tick)
> @@ -4433,10 +4434,11 @@ void update_cpu_load_nohz(void)
>  	if (pending_updates) {
>  		this_rq->last_load_update_tick = curr_jiffies;
>  		/*
> -		 * We were idle, this means load 0, the current load might be
> -		 * !0 due to remote wakeups and the sort.
> +		 * In the regular NOHZ case, we were idle, this means load 0.
> +		 * In the NOHZ_FULL case, we were non-idle, we should consider
> +		 * its weighted load.
>  		 */
> -		__update_cpu_load(this_rq, 0, pending_updates, 0);
> +		__update_cpu_load(this_rq, load, pending_updates, active);
>  	}
>  	raw_spin_unlock(&this_rq->lock);
>  }

Bah, so I did all the work to get the actual number of lost ticks in
there, only to _then_ find out that's mostly pointless :-)

The problem is update_idle_cpu_load() is called while idle (from another
CPU), so it still needs the whole jiffy based thing.

So I'll take this patch for now. Thanks.

Re: [PATCH v5 2/2] sched: make update_cpu_load_nohz() consider missed ticks in NOHZ_FULL

[Byungchul Park]

On Fri, Nov 20, 2015 at 02:19:55PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 10, 2015 at 09:36:02AM +0900, byungchul.park@lge.com wrote:
> > +++ b/kernel/sched/fair.c
> > @@ -4419,10 +4419,11 @@ static void update_idle_cpu_load(struct rq *this_rq)
> >  /*
> >   * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
> >   */
> > -void update_cpu_load_nohz(void)
> > +void update_cpu_load_nohz(int active)
> >  {
> >  	struct rq *this_rq = this_rq();
> >  	unsigned long curr_jiffies = READ_ONCE(jiffies);
> > +	unsigned long load = active ? weighted_cpuload(cpu_of(this_rq)) : 0;
> >  	unsigned long pending_updates;
> >  
> >  	if (curr_jiffies == this_rq->last_load_update_tick)
> > @@ -4433,10 +4434,11 @@ void update_cpu_load_nohz(void)
> >  	if (pending_updates) {
> >  		this_rq->last_load_update_tick = curr_jiffies;
> >  		/*
> > -		 * We were idle, this means load 0, the current load might be
> > -		 * !0 due to remote wakeups and the sort.
> > +		 * In the regular NOHZ case, we were idle, this means load 0.
> > +		 * In the NOHZ_FULL case, we were non-idle, we should consider
> > +		 * its weighted load.
> >  		 */
> > -		__update_cpu_load(this_rq, 0, pending_updates, 0);
> > +		__update_cpu_load(this_rq, load, pending_updates, active);
> >  	}
> >  	raw_spin_unlock(&this_rq->lock);
> >  }
> 
> Bah, so I did all the work to get the actual number of lost ticks in
> there, only to _then_ find out that's mostly pointless :-)
> 
> The problem is update_idle_cpu_load() is called while idle (from another
> CPU), so it still needs the whole jiffy based thing.
> 
> So I'll take this patch for now. Thanks.

Thank you.

Byungchul

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[tip:sched/core] sched/fair: Consider missed ticks in NOHZ_FULL in update_cpu_load_nohz()

[tip-bot for Byungchul Park]

Commit-ID:  525705d15e63b7455977408e4601e76e6bc41524
Gitweb:     http://git.kernel.org/tip/525705d15e63b7455977408e4601e76e6bc41524
Author:     Byungchul Park <byungchul.park@lge.com>
AuthorDate: Tue, 10 Nov 2015 09:36:02 +0900
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 23 Nov 2015 09:37:53 +0100

sched/fair: Consider missed ticks in NOHZ_FULL in update_cpu_load_nohz()

Usually the tick can be stopped for an idle CPU in NOHZ. However in NOHZ_FULL
mode, a non-idle CPU's tick can also be stopped. However, update_cpu_load_nohz()
does not consider the case a non-idle CPU's tick has been stopped at all.

This patch makes the update_cpu_load_nohz() know if the calling path comes
from NOHZ_FULL or idle NOHZ.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447115762-19734-3-git-send-email-byungchul.park@lge.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/sched.h    |  4 ++--
 kernel/sched/fair.c      | 10 ++++++----
 kernel/time/tick-sched.c |  8 ++++----
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index edad7a4..f425aac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -177,9 +177,9 @@ extern void get_iowait_load(unsigned long *nr_waiters, unsigned long *load);
 extern void calc_global_load(unsigned long ticks);
 
 #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
-extern void update_cpu_load_nohz(void);
+extern void update_cpu_load_nohz(int active);
 #else
-static inline void update_cpu_load_nohz(void) { }
+static inline void update_cpu_load_nohz(int active) { }
 #endif
 
 extern unsigned long get_parent_ip(unsigned long addr);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 404006a..309b1d5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4397,10 +4397,11 @@ static void update_idle_cpu_load(struct rq *this_rq)
 /*
  * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
  */
-void update_cpu_load_nohz(void)
+void update_cpu_load_nohz(int active)
 {
 	struct rq *this_rq = this_rq();
 	unsigned long curr_jiffies = READ_ONCE(jiffies);
+	unsigned long load = active ? weighted_cpuload(cpu_of(this_rq)) : 0;
 	unsigned long pending_updates;
 
 	if (curr_jiffies == this_rq->last_load_update_tick)
@@ -4411,10 +4412,11 @@ void update_cpu_load_nohz(void)
 	if (pending_updates) {
 		this_rq->last_load_update_tick = curr_jiffies;
 		/*
-		 * We were idle, this means load 0, the current load might be
-		 * !0 due to remote wakeups and the sort.
+		 * In the regular NOHZ case, we were idle, this means load 0.
+		 * In the NOHZ_FULL case, we were non-idle, we should consider
+		 * its weighted load.
 		 */
-		__update_cpu_load(this_rq, 0, pending_updates, 0);
+		__update_cpu_load(this_rq, load, pending_updates, active);
 	}
 	raw_spin_unlock(&this_rq->lock);
 }
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7c7ec45..515edf3 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -694,11 +694,11 @@ out:
 	return tick;
 }
 
-static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
+static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now, int active)
 {
 	/* Update jiffies first */
 	tick_do_update_jiffies64(now);
-	update_cpu_load_nohz();
+	update_cpu_load_nohz(active);
 
 	calc_load_exit_idle();
 	touch_softlockup_watchdog();
@@ -725,7 +725,7 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
 	if (can_stop_full_tick())
 		tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
 	else if (ts->tick_stopped)
-		tick_nohz_restart_sched_tick(ts, ktime_get());
+		tick_nohz_restart_sched_tick(ts, ktime_get(), 1);
 #endif
 }
 
@@ -916,7 +916,7 @@ void tick_nohz_idle_exit(void)
 		tick_nohz_stop_idle(ts, now);
 
 	if (ts->tick_stopped) {
-		tick_nohz_restart_sched_tick(ts, now);
+		tick_nohz_restart_sched_tick(ts, now, 0);
 		tick_nohz_account_idle_ticks(ts);
 	}