[patch] sched: fix broken smt/mc optimizations with CFS

[patch] sched: fix broken smt/mc optimizations with CFS

[Siddha, Suresh B]

Ingo, let me know if there any side effects of this change. Thanks.
---

On a four package system with HT - HT load balancing optimizations
were broken. For example, if two tasks end up running on two logical
threads of one of the packages, scheduler is not able to pull one of
the tasks to a completely idle package.

In this scenario, for nice-0 tasks, imbalance calculated by scheduler
will be 512 and find_busiest_queue() will return 0 (as each cpu's load
is 1024 > imbalance and has only one task running).

Similarly MC scheduler optimizations also get fixed with this patch.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
---

diff --git a/kernel/sched.c b/kernel/sched.c
index 45e17b8..c5ac710 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2494,7 +2494,7 @@ group_next:
 	 * a think about bumping its value to force at least one task to be
 	 * moved
 	 */
-	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
+	if (*imbalance < busiest_load_per_task) {
 		unsigned long tmp, pwr_now, pwr_move;
 		unsigned int imbn;
 

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Ingo Molnar]

* Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:

> Ingo, let me know if there any side effects of this change. Thanks.
> ---
> 
> On a four package system with HT - HT load balancing optimizations 
> were broken. For example, if two tasks end up running on two logical 
> threads of one of the packages, scheduler is not able to pull one of 
> the tasks to a completely idle package.

thanks, i've applied your fix to my queue.

	Ingo

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Ingo Molnar]

* Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:

>  	 * a think about bumping its value to force at least one task to be
>  	 * moved
>  	 */
> -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
> +	if (*imbalance < busiest_load_per_task) {
>  		unsigned long tmp, pwr_now, pwr_move;

hm, found a problem: this removes the 'fuzz' from balancing, which is a 
slight over-balancing to perturb CPU-bound tasks to be distributed in a 
fairer manner between CPUs. So how about the patch below instead?

a good testcase for this is to start 3 CPU-bound tasks on a 2-core box:

  for ((i=0; i<3; i++)); do while :; do :; done & done

with your patch applied two of the loops stick to one core, getting 50% 
each - the third loop sticks to the other core, getting 100% CPU time:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 3093 root      20   0  2736  528  252 R  100  0.0   0:23.81 bash
 3094 root      20   0  2736  532  256 R   50  0.0   0:11.95 bash
 3095 root      20   0  2736  532  256 R   50  0.0   0:11.95 bash

with no patch, or with my patch below each gets ~66% of CPU time, 
long-term:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 2290 mingo     20   0  2736  528  252 R   67  0.0   3:22.95 bash
 2291 mingo     20   0  2736  532  256 R   67  0.0   3:18.94 bash
 2292 mingo     20   0  2736  532  256 R   66  0.0   3:19.83 bash

the breakage wasnt caused by the fuzz, it was caused by the /2 - the 
patch below should fix this for real.

	Ingo

------------------------------>
Subject: sched: fix broken SMT/MC optimizations
From: "Siddha, Suresh B" <suresh.b.siddha@intel.com>

On a four package system with HT - HT load balancing optimizations were 
broken.  For example, if two tasks end up running on two logical threads 
of one of the packages, scheduler is not able to pull one of the tasks 
to a completely idle package.

In this scenario, for nice-0 tasks, imbalance calculated by scheduler 
will be 512 and find_busiest_queue() will return 0 (as each cpu's load 
is 1024 > imbalance and has only one task running).

Similarly MC scheduler optimizations also get fixed with this patch.

[ mingo@elte.hu: restored fair balancing by increasing the fuzz and
                 adding it back to the power decision, without the /2
                 factor. ]

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---

 include/linux/sched.h |    2 +-
 kernel/sched.c        |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -681,7 +681,7 @@ enum cpu_idle_type {
 #define SCHED_LOAD_SHIFT	10
 #define SCHED_LOAD_SCALE	(1L << SCHED_LOAD_SHIFT)
 
-#define SCHED_LOAD_SCALE_FUZZ	(SCHED_LOAD_SCALE >> 1)
+#define SCHED_LOAD_SCALE_FUZZ	SCHED_LOAD_SCALE
 
 #ifdef CONFIG_SMP
 #define SD_LOAD_BALANCE		1	/* Do load balancing on this domain. */
Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -2517,7 +2517,7 @@ group_next:
 	 * a think about bumping its value to force at least one task to be
 	 * moved
 	 */
-	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
+	if (*imbalance < busiest_load_per_task) {
 		unsigned long tmp, pwr_now, pwr_move;
 		unsigned int imbn;
 

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Ingo Molnar]

* Ingo Molnar <mingo@elte.hu> wrote:

> [...] So how about the patch below instead?

the right patch attached.

-------------------------------->
Subject: sched: fix broken SMT/MC optimizations
From: "Siddha, Suresh B" <suresh.b.siddha@intel.com>

On a four package system with HT - HT load balancing optimizations were
broken.  For example, if two tasks end up running on two logical threads
of one of the packages, scheduler is not able to pull one of the tasks
to a completely idle package.

In this scenario, for nice-0 tasks, imbalance calculated by scheduler
will be 512 and find_busiest_queue() will return 0 (as each cpu's load
is 1024 > imbalance and has only one task running).

Similarly MC scheduler optimizations also get fixed with this patch.

[ mingo@elte.hu: restored fair balancing by increasing the fuzz and
                 adding it back to the power decision, without the /2
                 factor. ]

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---

 include/linux/sched.h |    2 +-
 kernel/sched.c        |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -681,7 +681,7 @@ enum cpu_idle_type {
 #define SCHED_LOAD_SHIFT	10
 #define SCHED_LOAD_SCALE	(1L << SCHED_LOAD_SHIFT)
 
-#define SCHED_LOAD_SCALE_FUZZ	(SCHED_LOAD_SCALE >> 1)
+#define SCHED_LOAD_SCALE_FUZZ	SCHED_LOAD_SCALE
 
 #ifdef CONFIG_SMP
 #define SD_LOAD_BALANCE		1	/* Do load balancing on this domain. */
Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -2517,7 +2517,7 @@ group_next:
 	 * a think about bumping its value to force at least one task to be
 	 * moved
 	 */
-	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
+	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
 		unsigned long tmp, pwr_now, pwr_move;
 		unsigned int imbn;
 

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Daniel Walker]

On Thu, 2007-08-23 at 14:13 +0200, Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
> > [...] So how about the patch below instead?
> 
> the right patch attached.
> 
> -------------------------------->
> Subject: sched: fix broken SMT/MC optimizations
> From: "Siddha, Suresh B" <suresh.b.siddha@intel.com>
> 
> On a four package system with HT - HT load balancing optimizations were
> broken.  For example, if two tasks end up running on two logical threads
> of one of the packages, scheduler is not able to pull one of the tasks
> to a completely idle package.
> 
> In this scenario, for nice-0 tasks, imbalance calculated by scheduler
> will be 512 and find_busiest_queue() will return 0 (as each cpu's load
> is 1024 > imbalance and has only one task running).

Is there an upper bound on the number of tasks that can migrate during a
new idle balance? The reason that I'm asking is because I've been seeing
some large latency in the new idle path into load_balance().. I'm not
sure if lots of tasks are getting migrated or if it's just the iteration
over tasks in the rb-tree ..

Daniel

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Siddha, Suresh B]

On Thu, Aug 23, 2007 at 02:13:41PM +0200, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
> > [...] So how about the patch below instead?
> 
> the right patch attached.
> 
> -------------------------------->
> Subject: sched: fix broken SMT/MC optimizations
> From: "Siddha, Suresh B" <suresh.b.siddha@intel.com>
> 
> On a four package system with HT - HT load balancing optimizations were
> broken.  For example, if two tasks end up running on two logical threads
> of one of the packages, scheduler is not able to pull one of the tasks
> to a completely idle package.
> 
> In this scenario, for nice-0 tasks, imbalance calculated by scheduler
> will be 512 and find_busiest_queue() will return 0 (as each cpu's load
> is 1024 > imbalance and has only one task running).
> 
> Similarly MC scheduler optimizations also get fixed with this patch.
> 
> [ mingo@elte.hu: restored fair balancing by increasing the fuzz and
>                  adding it back to the power decision, without the /2
>                  factor. ]
> 
> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> ---
> 
>  include/linux/sched.h |    2 +-
>  kernel/sched.c        |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> Index: linux/include/linux/sched.h
> ===================================================================
> --- linux.orig/include/linux/sched.h
> +++ linux/include/linux/sched.h
> @@ -681,7 +681,7 @@ enum cpu_idle_type {
>  #define SCHED_LOAD_SHIFT	10
>  #define SCHED_LOAD_SCALE	(1L << SCHED_LOAD_SHIFT)
>  
> -#define SCHED_LOAD_SCALE_FUZZ	(SCHED_LOAD_SCALE >> 1)
> +#define SCHED_LOAD_SCALE_FUZZ	SCHED_LOAD_SCALE
>  
>  #ifdef CONFIG_SMP
>  #define SD_LOAD_BALANCE		1	/* Do load balancing on this domain. */
> Index: linux/kernel/sched.c
> ===================================================================
> --- linux.orig/kernel/sched.c
> +++ linux/kernel/sched.c
> @@ -2517,7 +2517,7 @@ group_next:
>  	 * a think about bumping its value to force at least one task to be
>  	 * moved
>  	 */
> -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
> +	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {

Ingo, this is still broken. This condition is always false for nice-0 tasks..

thanks,
suresh

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Ingo Molnar]

* Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:

> > -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
> > +	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
> 
> Ingo, this is still broken. This condition is always false for nice-0 
> tasks..

yes - negative reniced tasks are not spread out via this - and positive 
reniced tasks are spread out more easily.

	Ingo

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Siddha, Suresh B]

On Mon, Aug 27, 2007 at 09:23:24PM +0200, Ingo Molnar wrote:
> 
> * Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:
> 
> > > -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
> > > +	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
> > 
> > Ingo, this is still broken. This condition is always false for nice-0 
> > tasks..
> 
> yes - negative reniced tasks are not spread out via this - and positive 
> reniced tasks are spread out more easily.

Or the opposite?

Essentially I observed that nice 0 tasks still endup on two cores of same
package, with out getting spread out to two different packages. This behavior
is same with out this fix and this fix doesn't help in any way.

thanks,
suresh

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Siddha, Suresh B]

On Mon, Aug 27, 2007 at 12:31:03PM -0700, Siddha, Suresh B wrote:
> Essentially I observed that nice 0 tasks still endup on two cores of same
> package, with out getting spread out to two different packages. This behavior
> is same with out this fix and this fix doesn't help in any way.

Ingo, Appended patch seems to fix the issue and as far as I can test, seems ok
to me.

This is a quick fix for .23. Peter Williams and myself plan to look at
code cleanups in this area (HT/MC optimizations) post .23

BTW, with this fix, do you want to retain the current FUZZ value?

thanks,
suresh
--

Try to fix MC/HT scheduler optimization breakage again, with out breaking
the FUZZ logic.

First fix the check
	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task)
with this
	if (*imbalance < busiest_load_per_task)

As the current check is always false for nice 0 tasks (as SCHED_LOAD_SCALE_FUZZ
is same as busiest_load_per_task for nice 0 tasks).

With the above change, imbalance was getting reset to 0 in the corner case
condition, making the FUZZ logic fail. Fix it by not corrupting the
imbalance and change the imbalance, only when it finds that the
HT/MC optimization is needed.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
---

diff --git a/kernel/sched.c b/kernel/sched.c
index 9fe473a..03e5e8d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2511,7 +2511,7 @@ group_next:
 	 * a think about bumping its value to force at least one task to be
 	 * moved
 	 */
-	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
+	if (*imbalance < busiest_load_per_task) {
 		unsigned long tmp, pwr_now, pwr_move;
 		unsigned int imbn;
 
@@ -2563,10 +2563,8 @@ small_imbalance:
 		pwr_move /= SCHED_LOAD_SCALE;
 
 		/* Move if we gain throughput */
-		if (pwr_move <= pwr_now)
-			goto out_balanced;
-
-		*imbalance = busiest_load_per_task;
+		if (pwr_move > pwr_now)
+			*imbalance = busiest_load_per_task;
 	}
 
 	return busiest;

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Ingo Molnar]

* Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:

> On Mon, Aug 27, 2007 at 12:31:03PM -0700, Siddha, Suresh B wrote:
> > Essentially I observed that nice 0 tasks still endup on two cores of same
> > package, with out getting spread out to two different packages. This behavior
> > is same with out this fix and this fix doesn't help in any way.
> 
> Ingo, Appended patch seems to fix the issue and as far as I can test, 
> seems ok to me.

thanks! I've queued your fix up for .23 merge. I've done a quick test 
and it indeed seems to work well.

> This is a quick fix for .23. Peter Williams and myself plan to look at 
> code cleanups in this area (HT/MC optimizations) post .23
> 
> BTW, with this fix, do you want to retain the current FUZZ value?

what value would you suggest? I was thinking about using 
busiest_rq->curr->load.weight instead, to always keep rotating tasks.

	Ingo

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Chuck Ebbert]

On 08/28/2007 06:27 PM, Siddha, Suresh B wrote:
> On Mon, Aug 27, 2007 at 12:31:03PM -0700, Siddha, Suresh B wrote:
>> Essentially I observed that nice 0 tasks still endup on two cores of same
>> package, with out getting spread out to two different packages. This behavior
>> is same with out this fix and this fix doesn't help in any way.
> 
> Ingo, Appended patch seems to fix the issue and as far as I can test, seems ok
> to me.
> 
> This is a quick fix for .23. Peter Williams and myself plan to look at
> code cleanups in this area (HT/MC optimizations) post .23
> 
> BTW, with this fix, do you want to retain the current FUZZ value?
> 
> thanks,
> suresh
> --
> 
> Try to fix MC/HT scheduler optimization breakage again, with out breaking
> the FUZZ logic.
> 
> First fix the check
> 	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task)
> with this
> 	if (*imbalance < busiest_load_per_task)
> 
> As the current check is always false for nice 0 tasks (as SCHED_LOAD_SCALE_FUZZ
> is same as busiest_load_per_task for nice 0 tasks).
> 
> With the above change, imbalance was getting reset to 0 in the corner case
> condition, making the FUZZ logic fail. Fix it by not corrupting the
> imbalance and change the imbalance, only when it finds that the
> HT/MC optimization is needed.
> 
> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
> ---
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 9fe473a..03e5e8d 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2511,7 +2511,7 @@ group_next:
>  	 * a think about bumping its value to force at least one task to be
>  	 * moved
>  	 */
> -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
> +	if (*imbalance < busiest_load_per_task) {
>  		unsigned long tmp, pwr_now, pwr_move;
>  		unsigned int imbn;
>  
> @@ -2563,10 +2563,8 @@ small_imbalance:
>  		pwr_move /= SCHED_LOAD_SCALE;
>  
>  		/* Move if we gain throughput */
> -		if (pwr_move <= pwr_now)
> -			goto out_balanced;
> -
> -		*imbalance = busiest_load_per_task;
> +		if (pwr_move > pwr_now)
> +			*imbalance = busiest_load_per_task;
>  	}
>  
>  	return busiest;

Seems this didn't get merged? Latest git as of today still has the code
as it was before this patch.

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Siddha, Suresh B]

On Tue, Sep 04, 2007 at 07:35:21PM -0400, Chuck Ebbert wrote:
> On 08/28/2007 06:27 PM, Siddha, Suresh B wrote:
> > Try to fix MC/HT scheduler optimization breakage again, with out breaking
> > the FUZZ logic.
> > 
> > First fix the check
> > 	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task)
> > with this
> > 	if (*imbalance < busiest_load_per_task)
> > 
> > As the current check is always false for nice 0 tasks (as SCHED_LOAD_SCALE_FUZZ
> > is same as busiest_load_per_task for nice 0 tasks).
> > 
> > With the above change, imbalance was getting reset to 0 in the corner case
> > condition, making the FUZZ logic fail. Fix it by not corrupting the
> > imbalance and change the imbalance, only when it finds that the
> > HT/MC optimization is needed.
> > 
> > Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
> > ---
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 9fe473a..03e5e8d 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -2511,7 +2511,7 @@ group_next:
> >  	 * a think about bumping its value to force at least one task to be
> >  	 * moved
> >  	 */
> > -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
> > +	if (*imbalance < busiest_load_per_task) {
> >  		unsigned long tmp, pwr_now, pwr_move;
> >  		unsigned int imbn;
> >  
> > @@ -2563,10 +2563,8 @@ small_imbalance:
> >  		pwr_move /= SCHED_LOAD_SCALE;
> >  
> >  		/* Move if we gain throughput */
> > -		if (pwr_move <= pwr_now)
> > -			goto out_balanced;
> > -
> > -		*imbalance = busiest_load_per_task;
> > +		if (pwr_move > pwr_now)
> > +			*imbalance = busiest_load_per_task;
> >  	}
> >  
> >  	return busiest;
> 
> Seems this didn't get merged? Latest git as of today still has the code
> as it was before this patch.

This is must fix for .23 and Ingo previously mentioned that he will push it
for .23

Ingo?

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Ingo Molnar]

* Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:

> > Seems this didn't get merged? Latest git as of today still has the code
> > as it was before this patch.
> 
> This is must fix for .23 and Ingo previously mentioned that he will push it
> for .23

yep, it's queued up and it will send it with the next batch. (this is 
the most important scheduler fix we have at the moment - there are 3 
other, smaller items queued up as well)

	Ingo

Re: [patch] sched: fix broken smt/mc optimizations with CFS

[Torsten Kaiser]

On 8/23/07, Ingo Molnar <mingo@elte.hu> wrote:
> with no patch, or with my patch below each gets ~66% of CPU time,
> long-term:
>
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  2290 mingo     20   0  2736  528  252 R   67  0.0   3:22.95 bash
>  2291 mingo     20   0  2736  532  256 R   67  0.0   3:18.94 bash
>  2292 mingo     20   0  2736  532  256 R   66  0.0   3:19.83 bash
>

I just witnessed another scheduling "bug" that might have been a feature.
I use the current 2.6.23-rc3-mm1 kernel without any additional patches.

I have a 2x2218 Opteron system using the ondemand cpufreq governor,
one CPU was a max 2600 MHz, the other was at 1000 MHz.
On this system there were three processes (all niced) running, but
they all ended up at one CPU package, so that the distribution was
100-50-50 and the other CPU still idle.

So while the 100-50-50 distribution on one CPU might be fixed by your
patch, I am interested if the behavior that the second CPU remained
idle was intended.

On one hand it made perfectly sense: Even if one 50% task would be
migrated it would one get 1000MHz of CPU before the ondemand governor
kicked in, instead of 50% of 2600MHz == 1300MHZ.

A quick grep did not show me any references to cpufreq or governors in
kernel/sched* so I would expect that the scheduler can not predict
that the CPU will power up, if a task will be migrated there.

Part of my config:
CONFIG_NO_HZ=y
CONFIG_HIGH_RES_TIMERS=y
...
# CONFIG_SCHED_SMT is not set
CONFIG_SCHED_MC=y
CONFIG_PREEMPT_NONE=y
# CONFIG_PREEMPT_VOLUNTARY is not set
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_BKL=y
CONFIG_PREEMPT_NOTIFIERS=y
...
CONFIG_HZ_100=y
# CONFIG_HZ_250 is not set
# CONFIG_HZ_300 is not set
# CONFIG_HZ_1000 is not set
CONFIG_HZ=100
...
# CONFIG_SCHED_DEBUG is not set
# CONFIG_SCHEDSTATS is not set

My testcase is not reproducibly as it happened, but I could try to
recreate this, if it is necessary.

(I was running the screen saver from electricsheep.org and the three
niced tasks were three of its render threads)

Torsten