vmstat: make vmstat_updater deferrable again and shut down on idle

vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

Currently the vmstat updater is not deferrable as a result of commit
ba4877b9ca51f80b5d30f304a46762f0509e1635. This in turn can cause multiple
interruptions of the applications because the vmstat updater may run at
different times than tick processing. No good.

Make vmstate_update deferrable again and provide a function that
folds the differentials when the processor is going to idle mode thus
addressing the issue of the above commit in a clean way.

Note that the shepherd thread will continue scanning the differentials
from another processor and will reenable the vmstat workers if it
detects any changes.

Fixes: ba4877b9ca51f80b5d30f304a46762f0509e1635 (do not use deferrable delay)
Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/vmstat.c
===================================================================
--- linux.orig/mm/vmstat.c
+++ linux/mm/vmstat.c
@@ -460,7 +460,7 @@ static int fold_diff(int *diff)
  *
  * The function returns the number of global counters updated.
  */
-static int refresh_cpu_vm_stats(void)
+static int refresh_cpu_vm_stats(bool do_pagesets)
 {
 	struct zone *zone;
 	int i;
@@ -484,33 +484,35 @@ static int refresh_cpu_vm_stats(void)
 #endif
 			}
 		}
-		cond_resched();
 #ifdef CONFIG_NUMA
-		/*
-		 * Deal with draining the remote pageset of this
-		 * processor
-		 *
-		 * Check if there are pages remaining in this pageset
-		 * if not then there is nothing to expire.
-		 */
-		if (!__this_cpu_read(p->expire) ||
+		if (do_pagesets) {
+			cond_resched();
+			/*
+			 * Deal with draining the remote pageset of this
+			 * processor
+			 *
+			 * Check if there are pages remaining in this pageset
+			 * if not then there is nothing to expire.
+			 */
+			if (!__this_cpu_read(p->expire) ||
 			       !__this_cpu_read(p->pcp.count))
-			continue;
+				continue;

-		/*
-		 * We never drain zones local to this processor.
-		 */
-		if (zone_to_nid(zone) == numa_node_id()) {
-			__this_cpu_write(p->expire, 0);
-			continue;
-		}
+			/*
+			 * We never drain zones local to this processor.
+			 */
+			if (zone_to_nid(zone) == numa_node_id()) {
+				__this_cpu_write(p->expire, 0);
+				continue;
+			}

-		if (__this_cpu_dec_return(p->expire))
-			continue;
+			if (__this_cpu_dec_return(p->expire))
+				continue;

-		if (__this_cpu_read(p->pcp.count)) {
-			drain_zone_pages(zone, this_cpu_ptr(&p->pcp));
-			changes++;
+			if (__this_cpu_read(p->pcp.count)) {
+				drain_zone_pages(zone, this_cpu_ptr(&p->pcp));
+				changes++;
+			}
 		}
 #endif
 	}
@@ -1376,7 +1378,7 @@ static cpumask_var_t cpu_stat_off;

 static void vmstat_update(struct work_struct *w)
 {
-	if (refresh_cpu_vm_stats()) {
+	if (refresh_cpu_vm_stats(true)) {
 		/*
 		 * Counters were updated so we expect more updates
 		 * to occur in the future. Keep on running the
@@ -1408,6 +1410,20 @@ static void vmstat_update(struct work_st
 }

 /*
+ * Switch off vmstat processing and then fold all the remaining differentials
+ * until the diffs stay at zero. The function is used by NOHZ and can only be
+ * invoked when tick processing is not active.
+ */
+void quiet_vmstat(void)
+{
+	do {
+		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
+			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
+
+	} while (refresh_cpu_vm_stats(false));
+}
+
+/*
  * Check if the diffs for a certain cpu indicate that
  * an update is needed.
  */
@@ -1439,7 +1455,7 @@ static bool need_update(int cpu)
  */
 static void vmstat_shepherd(struct work_struct *w);

-static DECLARE_DELAYED_WORK(shepherd, vmstat_shepherd);
+static DECLARE_DEFERRABLE_WORK(shepherd, vmstat_shepherd);

 static void vmstat_shepherd(struct work_struct *w)
 {
Index: linux/include/linux/vmstat.h
===================================================================
--- linux.orig/include/linux/vmstat.h
+++ linux/include/linux/vmstat.h
@@ -189,6 +189,7 @@ extern void __inc_zone_state(struct zone
 extern void dec_zone_state(struct zone *, enum zone_stat_item);
 extern void __dec_zone_state(struct zone *, enum zone_stat_item);

+void quiet_vmstat(void);
 void cpu_vm_stats_fold(int cpu);
 void refresh_zone_stat_thresholds(void);

@@ -249,6 +250,7 @@ static inline void __dec_zone_page_state

 static inline void refresh_zone_stat_thresholds(void) { }
 static inline void cpu_vm_stats_fold(int cpu) { }
+static inline void quiet_vmstat(void) { }

 static inline void drain_zonestat(struct zone *zone,
 			struct per_cpu_pageset *pset) { }
Index: linux/kernel/sched/idle.c
===================================================================
--- linux.orig/kernel/sched/idle.c
+++ linux/kernel/sched/idle.c
@@ -219,6 +219,7 @@ static void cpu_idle_loop(void)
 		 */

 		__current_set_polling();
+		quiet_vmstat();
 		tick_nohz_idle_enter();

 		while (!need_resched()) {

vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

Currently the vmstat updater is not deferrable as a result of commit
ba4877b9ca51f80b5d30f304a46762f0509e1635. This in turn can cause multiple
interruptions of the applications because the vmstat updater may run at
different times than tick processing. No good.

Make vmstate_update deferrable again and provide a function that
folds the differentials when the processor is going to idle mode thus
addressing the issue of the above commit in a clean way.

Note that the shepherd thread will continue scanning the differentials
from another processor and will reenable the vmstat workers if it
detects any changes.

Fixes: ba4877b9ca51f80b5d30f304a46762f0509e1635 (do not use deferrable delay)
Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/vmstat.c
===================================================================
--- linux.orig/mm/vmstat.c
+++ linux/mm/vmstat.c
@@ -460,7 +460,7 @@ static int fold_diff(int *diff)
  *
  * The function returns the number of global counters updated.
  */
-static int refresh_cpu_vm_stats(void)
+static int refresh_cpu_vm_stats(bool do_pagesets)
 {
 	struct zone *zone;
 	int i;
@@ -484,33 +484,35 @@ static int refresh_cpu_vm_stats(void)
 #endif
 			}
 		}
-		cond_resched();
 #ifdef CONFIG_NUMA
-		/*
-		 * Deal with draining the remote pageset of this
-		 * processor
-		 *
-		 * Check if there are pages remaining in this pageset
-		 * if not then there is nothing to expire.
-		 */
-		if (!__this_cpu_read(p->expire) ||
+		if (do_pagesets) {
+			cond_resched();
+			/*
+			 * Deal with draining the remote pageset of this
+			 * processor
+			 *
+			 * Check if there are pages remaining in this pageset
+			 * if not then there is nothing to expire.
+			 */
+			if (!__this_cpu_read(p->expire) ||
 			       !__this_cpu_read(p->pcp.count))
-			continue;
+				continue;

-		/*
-		 * We never drain zones local to this processor.
-		 */
-		if (zone_to_nid(zone) == numa_node_id()) {
-			__this_cpu_write(p->expire, 0);
-			continue;
-		}
+			/*
+			 * We never drain zones local to this processor.
+			 */
+			if (zone_to_nid(zone) == numa_node_id()) {
+				__this_cpu_write(p->expire, 0);
+				continue;
+			}

-		if (__this_cpu_dec_return(p->expire))
-			continue;
+			if (__this_cpu_dec_return(p->expire))
+				continue;

-		if (__this_cpu_read(p->pcp.count)) {
-			drain_zone_pages(zone, this_cpu_ptr(&p->pcp));
-			changes++;
+			if (__this_cpu_read(p->pcp.count)) {
+				drain_zone_pages(zone, this_cpu_ptr(&p->pcp));
+				changes++;
+			}
 		}
 #endif
 	}
@@ -1376,7 +1378,7 @@ static cpumask_var_t cpu_stat_off;

 static void vmstat_update(struct work_struct *w)
 {
-	if (refresh_cpu_vm_stats()) {
+	if (refresh_cpu_vm_stats(true)) {
 		/*
 		 * Counters were updated so we expect more updates
 		 * to occur in the future. Keep on running the
@@ -1408,6 +1410,20 @@ static void vmstat_update(struct work_st
 }

 /*
+ * Switch off vmstat processing and then fold all the remaining differentials
+ * until the diffs stay at zero. The function is used by NOHZ and can only be
+ * invoked when tick processing is not active.
+ */
+void quiet_vmstat(void)
+{
+	do {
+		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
+			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
+
+	} while (refresh_cpu_vm_stats(false));
+}
+
+/*
  * Check if the diffs for a certain cpu indicate that
  * an update is needed.
  */
@@ -1439,7 +1455,7 @@ static bool need_update(int cpu)
  */
 static void vmstat_shepherd(struct work_struct *w);

-static DECLARE_DELAYED_WORK(shepherd, vmstat_shepherd);
+static DECLARE_DEFERRABLE_WORK(shepherd, vmstat_shepherd);

 static void vmstat_shepherd(struct work_struct *w)
 {
Index: linux/include/linux/vmstat.h
===================================================================
--- linux.orig/include/linux/vmstat.h
+++ linux/include/linux/vmstat.h
@@ -189,6 +189,7 @@ extern void __inc_zone_state(struct zone
 extern void dec_zone_state(struct zone *, enum zone_stat_item);
 extern void __dec_zone_state(struct zone *, enum zone_stat_item);

+void quiet_vmstat(void);
 void cpu_vm_stats_fold(int cpu);
 void refresh_zone_stat_thresholds(void);

@@ -249,6 +250,7 @@ static inline void __dec_zone_page_state

 static inline void refresh_zone_stat_thresholds(void) { }
 static inline void cpu_vm_stats_fold(int cpu) { }
+static inline void quiet_vmstat(void) { }

 static inline void drain_zonestat(struct zone *zone,
 			struct per_cpu_pageset *pset) { }
Index: linux/kernel/sched/idle.c
===================================================================
--- linux.orig/kernel/sched/idle.c
+++ linux/kernel/sched/idle.c
@@ -219,6 +219,7 @@ static void cpu_idle_loop(void)
 		 */

 		__current_set_polling();
+		quiet_vmstat();
 		tick_nohz_idle_enter();

 		while (!need_resched()) {

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Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Andrew Morton]

On Thu, 10 Dec 2015 14:45:02 -0600 (CST) Christoph Lameter <cl@linux.com> wrote:

> Currently the vmstat updater is not deferrable as a result of commit
> ba4877b9ca51f80b5d30f304a46762f0509e1635. This in turn can cause multiple
> interruptions of the applications because the vmstat updater may run at
> different times than tick processing. No good.
> 
> Make vmstate_update deferrable again and provide a function that
> folds the differentials when the processor is going to idle mode thus
> addressing the issue of the above commit in a clean way.
> 
> Note that the shepherd thread will continue scanning the differentials
> from another processor and will reenable the vmstat workers if it
> detects any changes.
> 
> Fixes: ba4877b9ca51f80b5d30f304a46762f0509e1635 (do not use deferrable delay)
> Signed-off-by: Christoph Lameter <cl@linux.com>
> 
> ...
>
>  /*
> + * Switch off vmstat processing and then fold all the remaining differentials
> + * until the diffs stay at zero. The function is used by NOHZ and can only be
> + * invoked when tick processing is not active.
> + */
> +void quiet_vmstat(void)
> +{
> +	do {
> +		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
> +			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
> +
> +	} while (refresh_cpu_vm_stats(false));
> +}

How do we know this will terminate in a reasonable amount of time if
other CPUs are pounding away?

Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Andrew Morton]

On Thu, 10 Dec 2015 14:45:02 -0600 (CST) Christoph Lameter <cl@linux.com> wrote:

> Currently the vmstat updater is not deferrable as a result of commit
> ba4877b9ca51f80b5d30f304a46762f0509e1635. This in turn can cause multiple
> interruptions of the applications because the vmstat updater may run at
> different times than tick processing. No good.
> 
> Make vmstate_update deferrable again and provide a function that
> folds the differentials when the processor is going to idle mode thus
> addressing the issue of the above commit in a clean way.
> 
> Note that the shepherd thread will continue scanning the differentials
> from another processor and will reenable the vmstat workers if it
> detects any changes.
> 
> Fixes: ba4877b9ca51f80b5d30f304a46762f0509e1635 (do not use deferrable delay)
> Signed-off-by: Christoph Lameter <cl@linux.com>
> 
> ...
>
>  /*
> + * Switch off vmstat processing and then fold all the remaining differentials
> + * until the diffs stay at zero. The function is used by NOHZ and can only be
> + * invoked when tick processing is not active.
> + */
> +void quiet_vmstat(void)
> +{
> +	do {
> +		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
> +			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
> +
> +	} while (refresh_cpu_vm_stats(false));
> +}

How do we know this will terminate in a reasonable amount of time if
other CPUs are pounding away?


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Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

On Thu, 10 Dec 2015, Andrew Morton wrote:

> >  /*
> > + * Switch off vmstat processing and then fold all the remaining differentials
> > + * until the diffs stay at zero. The function is used by NOHZ and can only be
> > + * invoked when tick processing is not active.
> > + */
> > +void quiet_vmstat(void)
> > +{
> > +	do {
> > +		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
> > +			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
> > +
> > +	} while (refresh_cpu_vm_stats(false));
> > +}
>
> How do we know this will terminate in a reasonable amount of time if
> other CPUs are pounding away?

This is only dealing with the differentials of the local cpu. Other cpus
do not matter.

Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

On Thu, 10 Dec 2015, Andrew Morton wrote:

> >  /*
> > + * Switch off vmstat processing and then fold all the remaining differentials
> > + * until the diffs stay at zero. The function is used by NOHZ and can only be
> > + * invoked when tick processing is not active.
> > + */
> > +void quiet_vmstat(void)
> > +{
> > +	do {
> > +		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
> > +			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
> > +
> > +	} while (refresh_cpu_vm_stats(false));
> > +}
>
> How do we know this will terminate in a reasonable amount of time if
> other CPUs are pounding away?

This is only dealing with the differentials of the local cpu. Other cpus
do not matter.

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Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

Hmmm... We got a race condition since quiet_vmstat touches cpu_stat_off
which may not be allocated early in the bootup sequence. Causes oopses on
boot.



Subject: vmstat: quieting vmstat requires a running system

Do not do anything unless the system is actually running. Otherwise
we may crash on bootup.

Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/vmstat.c
===================================================================
--- linux.orig/mm/vmstat.c
+++ linux/mm/vmstat.c
@@ -1270,6 +1270,9 @@ static void vmstat_update(struct work_st
  */
 void quiet_vmstat(void)
 {
+	if (system_state != SYSTEM_RUNNING)
+		return;
+
 	do {
 		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
 			cancel_delayed_work(this_cpu_ptr(&vmstat_work));

Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

Hmmm... We got a race condition since quiet_vmstat touches cpu_stat_off
which may not be allocated early in the bootup sequence. Causes oopses on
boot.



Subject: vmstat: quieting vmstat requires a running system

Do not do anything unless the system is actually running. Otherwise
we may crash on bootup.

Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/vmstat.c
===================================================================
--- linux.orig/mm/vmstat.c
+++ linux/mm/vmstat.c
@@ -1270,6 +1270,9 @@ static void vmstat_update(struct work_st
  */
 void quiet_vmstat(void)
 {
+	if (system_state != SYSTEM_RUNNING)
+		return;
+
 	do {
 		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
 			cancel_delayed_work(this_cpu_ptr(&vmstat_work));

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Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Shiraz Hashim]

Hi Christoph,

On Fri, Dec 11, 2015 at 2:15 AM, Christoph Lameter <cl@linux.com> wrote:
> Currently the vmstat updater is not deferrable as a result of commit
> ba4877b9ca51f80b5d30f304a46762f0509e1635. This in turn can cause multiple
> interruptions of the applications because the vmstat updater may run at
> different times than tick processing. No good.
>
> Make vmstate_update deferrable again and provide a function that
> folds the differentials when the processor is going to idle mode thus
> addressing the issue of the above commit in a clean way.
>

The patch makes vmstat_shepherd deferable which if is quiesed
would not schedule vmstat update on other cpus. Wouldn't this
aggravate the problem of vmstat for rest cpus not gettng updated.

regards
Shiraz

Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Shiraz Hashim]

Hi Christoph,

On Fri, Dec 11, 2015 at 2:15 AM, Christoph Lameter <cl@linux.com> wrote:
> Currently the vmstat updater is not deferrable as a result of commit
> ba4877b9ca51f80b5d30f304a46762f0509e1635. This in turn can cause multiple
> interruptions of the applications because the vmstat updater may run at
> different times than tick processing. No good.
>
> Make vmstate_update deferrable again and provide a function that
> folds the differentials when the processor is going to idle mode thus
> addressing the issue of the above commit in a clean way.
>

The patch makes vmstat_shepherd deferable which if is quiesed
would not schedule vmstat update on other cpus. Wouldn't this
aggravate the problem of vmstat for rest cpus not gettng updated.

regards
Shiraz

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Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

On Wed, 20 Jan 2016, Shiraz Hashim wrote:

> The patch makes vmstat_shepherd deferable which if is quiesed
> would not schedule vmstat update on other cpus. Wouldn't this
> aggravate the problem of vmstat for rest cpus not gettng updated.

Its only "deferred" in order to make it at the next tick and not cause an
extra event. This means that vmstat will run periodically from tick
processing. It merely causes a synching so that we have one interruption
that does both.

On idle we fold counters immediately. So there is no loss of accuracy.

Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

On Wed, 20 Jan 2016, Shiraz Hashim wrote:

> The patch makes vmstat_shepherd deferable which if is quiesed
> would not schedule vmstat update on other cpus. Wouldn't this
> aggravate the problem of vmstat for rest cpus not gettng updated.

Its only "deferred" in order to make it at the next tick and not cause an
extra event. This means that vmstat will run periodically from tick
processing. It merely causes a synching so that we have one interruption
that does both.

On idle we fold counters immediately. So there is no loss of accuracy.

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Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Shiraz Hashim]

On Wed, Jan 20, 2016 at 8:42 PM, Christoph Lameter <cl@linux.com> wrote:
> On Wed, 20 Jan 2016, Shiraz Hashim wrote:
>
>> The patch makes vmstat_shepherd deferable which if is quiesed
>> would not schedule vmstat update on other cpus. Wouldn't this
>> aggravate the problem of vmstat for rest cpus not gettng updated.
>
> Its only "deferred" in order to make it at the next tick and not cause an
> extra event. This means that vmstat will run periodically from tick
> processing. It merely causes a synching so that we have one interruption
> that does both.
>
> On idle we fold counters immediately. So there is no loss of accuracy.
>

vmstat is scheduled by shepherd or by itself (conditionally). In case shepherd
is deferred and vmstat doesn't schedule itself, then vmstat needs to wait
for shepherd to be up and then schedule it. This may end up in delayed status
update for all live cpus. Isn't it ?

-- 
regards
Shiraz Hashim

Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Shiraz Hashim]

On Wed, Jan 20, 2016 at 8:42 PM, Christoph Lameter <cl@linux.com> wrote:
> On Wed, 20 Jan 2016, Shiraz Hashim wrote:
>
>> The patch makes vmstat_shepherd deferable which if is quiesed
>> would not schedule vmstat update on other cpus. Wouldn't this
>> aggravate the problem of vmstat for rest cpus not gettng updated.
>
> Its only "deferred" in order to make it at the next tick and not cause an
> extra event. This means that vmstat will run periodically from tick
> processing. It merely causes a synching so that we have one interruption
> that does both.
>
> On idle we fold counters immediately. So there is no loss of accuracy.
>

vmstat is scheduled by shepherd or by itself (conditionally). In case shepherd
is deferred and vmstat doesn't schedule itself, then vmstat needs to wait
for shepherd to be up and then schedule it. This may end up in delayed status
update for all live cpus. Isn't it ?

-- 
regards
Shiraz Hashim

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Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

On Thu, 21 Jan 2016, Shiraz Hashim wrote:

> > On idle we fold counters immediately. So there is no loss of accuracy.
>
> vmstat is scheduled by shepherd or by itself (conditionally). In case shepherd
> is deferred and vmstat doesn't schedule itself, then vmstat needs to wait
> for shepherd to be up and then schedule it. This may end up in delayed status
> update for all live cpus. Isn't it ?

The shepherd runs on a processor with an active tick and thus should do
its duty every 2 seconds as scheduled. Small milisecond range deferrals do
not matter much.

Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Christoph Lameter]

On Thu, 21 Jan 2016, Shiraz Hashim wrote:

> > On idle we fold counters immediately. So there is no loss of accuracy.
>
> vmstat is scheduled by shepherd or by itself (conditionally). In case shepherd
> is deferred and vmstat doesn't schedule itself, then vmstat needs to wait
> for shepherd to be up and then schedule it. This may end up in delayed status
> update for all live cpus. Isn't it ?

The shepherd runs on a processor with an active tick and thus should do
its duty every 2 seconds as scheduled. Small milisecond range deferrals do
not matter much.

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