[2/2,v2] sched: use load_avg for selecting idlest group

Message ID 1480088073-11642-3-git-send-email-vincent.guittot@linaro.org
State Superseded
Headers show

Commit Message

Vincent Guittot Nov. 25, 2016, 3:34 p.m.
find_idlest_group() only compares the runnable_load_avg when looking for
the least loaded group. But on fork intensive use case like hackbench
where tasks blocked quickly after the fork, this can lead to selecting the
same CPU instead of other CPUs, which have similar runnable load but a
lower load_avg.

When the runnable_load_avg of 2 CPUs are close, we now take into account
the amount of blocked load as a 2nd selection factor. There is now 3 zones
for the runnable_load of the rq:
-[0 .. (runnable_load - imbalance)] : Select the new rq which has
significantly less runnable_load
-](runnable_load - imbalance) .. (runnable_load + imbalance)[ : The
runnable load are close so we use load_avg to chose between the 2 rq
-[(runnable_load + imbalance) .. ULONG_MAX] : Keep the current rq which
has significantly less runnable_load

For use case like hackbench, this enable the scheduler to select different
CPUs during the fork sequence and to spread tasks across the system.

Tests have been done on a Hikey board (ARM based octo cores) for several
kernel. The result below gives min, max, avg and stdev values of 18 runs
with each configuration.

The v4.8+patches configuration also includes the changes below which is
part of the proposal made by Peter to ensure that the clock will be up to
date when the fork task will be attached to the rq.

@@ -2568,6 +2568,7 @@ void wake_up_new_task(struct task_struct *p)
 	__set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
 #endif
 	rq = __task_rq_lock(p, &rf);
+	update_rq_clock(rq);
 	post_init_entity_util_avg(&p->se);

 	activate_task(rq, p, 0);

hackbench -P -g 1

       ea86cb4b7621  7dc603c9028e  v4.8        v4.8+patches
min    0.049         0.050         0.051       0,048
avg    0.057         0.057(0%)     0.057(0%)   0,055(+5%)
max    0.066         0.068         0.070       0,063
stdev  +/-9%         +/-9%         +/-8%       +/-9%

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>

---

Changes since v2:
- Rebase on latest sched/core
- Get same results with the rebase and the fix mentioned in patch 01

 kernel/sched/fair.c | 48 ++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 38 insertions(+), 10 deletions(-)

-- 
2.7.4

Comments

Morten Rasmussen Nov. 30, 2016, 12:49 p.m. | #1
On Fri, Nov 25, 2016 at 04:34:33PM +0100, Vincent Guittot wrote:
> find_idlest_group() only compares the runnable_load_avg when looking for

> the least loaded group. But on fork intensive use case like hackbench

> where tasks blocked quickly after the fork, this can lead to selecting the

> same CPU instead of other CPUs, which have similar runnable load but a

> lower load_avg.

> 

> When the runnable_load_avg of 2 CPUs are close, we now take into account

> the amount of blocked load as a 2nd selection factor. There is now 3 zones

> for the runnable_load of the rq:

> -[0 .. (runnable_load - imbalance)] : Select the new rq which has

> significantly less runnable_load

> -](runnable_load - imbalance) .. (runnable_load + imbalance)[ : The

> runnable load are close so we use load_avg to chose between the 2 rq

> -[(runnable_load + imbalance) .. ULONG_MAX] : Keep the current rq which

> has significantly less runnable_load

> 

> For use case like hackbench, this enable the scheduler to select different

> CPUs during the fork sequence and to spread tasks across the system.

> 

> Tests have been done on a Hikey board (ARM based octo cores) for several

> kernel. The result below gives min, max, avg and stdev values of 18 runs

> with each configuration.

> 

> The v4.8+patches configuration also includes the changes below which is

> part of the proposal made by Peter to ensure that the clock will be up to

> date when the fork task will be attached to the rq.

> 

> @@ -2568,6 +2568,7 @@ void wake_up_new_task(struct task_struct *p)

>  	__set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));

>  #endif

>  	rq = __task_rq_lock(p, &rf);

> +	update_rq_clock(rq);

>  	post_init_entity_util_avg(&p->se);

> 

>  	activate_task(rq, p, 0);

> 

> hackbench -P -g 1

> 

>        ea86cb4b7621  7dc603c9028e  v4.8        v4.8+patches

> min    0.049         0.050         0.051       0,048

> avg    0.057         0.057(0%)     0.057(0%)   0,055(+5%)

> max    0.066         0.068         0.070       0,063

> stdev  +/-9%         +/-9%         +/-8%       +/-9%

> 

> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---

> 

> Changes since v2:

> - Rebase on latest sched/core

> - Get same results with the rebase and the fix mentioned in patch 01

> 

>  kernel/sched/fair.c | 48 ++++++++++++++++++++++++++++++++++++++----------

>  1 file changed, 38 insertions(+), 10 deletions(-)

> 

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c

> index 820a787..ecb5ee8 100644

> --- a/kernel/sched/fair.c

> +++ b/kernel/sched/fair.c

> @@ -5395,16 +5395,20 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>  {

>  	struct sched_group *idlest = NULL, *group = sd->groups;

>  	struct sched_group *most_spare_sg = NULL;

> -	unsigned long min_load = ULONG_MAX, this_load = 0;

> +	unsigned long min_runnable_load = ULONG_MAX, this_runnable_load = 0;

> +	unsigned long min_avg_load = ULONG_MAX, this_avg_load = 0;

>  	unsigned long most_spare = 0, this_spare = 0;

>  	int load_idx = sd->forkexec_idx;

> -	int imbalance = 100 + (sd->imbalance_pct-100)/2;

> +	int imbalance_scale = 100 + (sd->imbalance_pct-100)/2;

> +	unsigned long imbalance = scale_load_down(NICE_0_LOAD) *

> +				(sd->imbalance_pct-100) / 100;

>  

>  	if (sd_flag & SD_BALANCE_WAKE)

>  		load_idx = sd->wake_idx;

>  

>  	do {

> -		unsigned long load, avg_load, spare_cap, max_spare_cap;

> +		unsigned long load, avg_load, runnable_load;

> +		unsigned long spare_cap, max_spare_cap;

>  		int local_group;

>  		int i;

>  

> @@ -5421,6 +5425,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>  		 * the group containing the CPU with most spare capacity.

>  		 */

>  		avg_load = 0;

> +		runnable_load = 0;

>  		max_spare_cap = 0;

>  

>  		for_each_cpu(i, sched_group_cpus(group)) {

> @@ -5430,7 +5435,9 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>  			else

>  				load = target_load(i, load_idx);

>  

> -			avg_load += load;

> +			runnable_load += load;

> +

> +			avg_load += cfs_rq_load_avg(&cpu_rq(i)->cfs);

>  

>  			spare_cap = capacity_spare_wake(i, p);

>  

> @@ -5439,14 +5446,32 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>  		}

>  

>  		/* Adjust by relative CPU capacity of the group */

> -		avg_load = (avg_load * SCHED_CAPACITY_SCALE) / group->sgc->capacity;

> +		avg_load = (avg_load * SCHED_CAPACITY_SCALE) /

> +					group->sgc->capacity;

> +		runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) /

> +					group->sgc->capacity;

>  

>  		if (local_group) {

> -			this_load = avg_load;

> +			this_runnable_load = runnable_load;

> +			this_avg_load = avg_load;

>  			this_spare = max_spare_cap;

>  		} else {

> -			if (avg_load < min_load) {

> -				min_load = avg_load;

> +			if (min_runnable_load > (runnable_load + imbalance)) {

> +				/*

> +				 * The runnable load is significantly smaller

> +				 *  so we can pick this new cpu

> +				 */

> +				min_runnable_load = runnable_load;

> +				min_avg_load = avg_load;

> +				idlest = group;

> +			} else if ((runnable_load < (min_runnable_load + imbalance)) &&

> +					(100*min_avg_load > imbalance_scale*avg_load)) {

> +				/*

> +				 * The runnable loads are close so we take

> +				 * into account blocked load through avg_load

> +				 *  which is blocked + runnable load

> +				 */

> +				min_avg_load = avg_load;

>  				idlest = group;

>  			}

>  

> @@ -5470,13 +5495,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>  		goto no_spare;

>  

>  	if (this_spare > task_util(p) / 2 &&

> -	    imbalance*this_spare > 100*most_spare)

> +	    imbalance_scale*this_spare > 100*most_spare)

>  		return NULL;

>  	else if (most_spare > task_util(p) / 2)

>  		return most_spare_sg;

>  

>  no_spare:

> -	if (!idlest || 100*this_load < imbalance*min_load)

> +	if (!idlest ||

> +	    (min_runnable_load > (this_runnable_load + imbalance)) ||

> +	    ((this_runnable_load < (min_runnable_load + imbalance)) &&

> +			(100*min_avg_load > imbalance_scale*this_avg_load)))


I don't get why you have imbalance_scale applied to this_avg_load and
not min_avg_load. IIUC, you end up preferring non-local groups?

If we take the example where this_runnable_load == min_runnable_load and
this_avg_load == min_avg_load. In this case, and in cases where
min_avg_load is slightly bigger than this_avg_load, we end up picking
the 'idlest' group even if the local group is equally good or even
slightly better?

>  		return NULL;

>  	return idlest;

>  }


Overall, I like that load_avg is being brought in to make better
decisions. The variable naming is a bit confusing. For example,
runnable_load is a capacity-average just like avg_load. 'imbalance' is
now an absolute capacity-average margin, but it is hard to come up with
better short alternatives.

Although 'imbalance' is based on the existing imbalance_pct, I find
somewhat arbitrary. Why is (imbalance_pct-100)*1024/100 a good absolute
margin to define the interval where we want to consider load_avg? I
guess it is case of 'we had to pick some value', which we have done in
many other places. Though, IMHO, it is a bit strange that imbalance_pct
is used in two different ways to bias comparison in the same function.
It used to be only used as a scaling factor (now imbalance_scale), while
this patch proposes to use it for computing an absolute margin
(imbalance) as well. It is not major issue, but it is not clear why it
is used differently to compare two metrics that are relatively closely
related.

Morten
Vincent Guittot Nov. 30, 2016, 1:49 p.m. | #2
On 30 November 2016 at 13:49, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> On Fri, Nov 25, 2016 at 04:34:33PM +0100, Vincent Guittot wrote:

>> find_idlest_group() only compares the runnable_load_avg when looking for

>> the least loaded group. But on fork intensive use case like hackbench


[snip]

>> +                             min_avg_load = avg_load;

>> +                             idlest = group;

>> +                     } else if ((runnable_load < (min_runnable_load + imbalance)) &&

>> +                                     (100*min_avg_load > imbalance_scale*avg_load)) {

>> +                             /*

>> +                              * The runnable loads are close so we take

>> +                              * into account blocked load through avg_load

>> +                              *  which is blocked + runnable load

>> +                              */

>> +                             min_avg_load = avg_load;

>>                               idlest = group;

>>                       }

>>

>> @@ -5470,13 +5495,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>>               goto no_spare;

>>

>>       if (this_spare > task_util(p) / 2 &&

>> -         imbalance*this_spare > 100*most_spare)

>> +         imbalance_scale*this_spare > 100*most_spare)

>>               return NULL;

>>       else if (most_spare > task_util(p) / 2)

>>               return most_spare_sg;

>>

>>  no_spare:

>> -     if (!idlest || 100*this_load < imbalance*min_load)

>> +     if (!idlest ||

>> +         (min_runnable_load > (this_runnable_load + imbalance)) ||

>> +         ((this_runnable_load < (min_runnable_load + imbalance)) &&

>> +                     (100*min_avg_load > imbalance_scale*this_avg_load)))

>

> I don't get why you have imbalance_scale applied to this_avg_load and

> not min_avg_load. IIUC, you end up preferring non-local groups?


In fact, I have keep the same condition that is used when looping the group.
You're right that we should prefer local rq if avg_load are close and
test the condition
(100*this_avg_load > imbalance_scale*min_avg_load) instead

>

> If we take the example where this_runnable_load == min_runnable_load and

> this_avg_load == min_avg_load. In this case, and in cases where

> min_avg_load is slightly bigger than this_avg_load, we end up picking

> the 'idlest' group even if the local group is equally good or even

> slightly better?

>

>>               return NULL;

>>       return idlest;

>>  }

>

> Overall, I like that load_avg is being brought in to make better

> decisions. The variable naming is a bit confusing. For example,

> runnable_load is a capacity-average just like avg_load. 'imbalance' is

> now an absolute capacity-average margin, but it is hard to come up with

> better short alternatives.

>

> Although 'imbalance' is based on the existing imbalance_pct, I find

> somewhat arbitrary. Why is (imbalance_pct-100)*1024/100 a good absolute

> margin to define the interval where we want to consider load_avg? I

> guess it is case of 'we had to pick some value', which we have done in

> many other places. Though, IMHO, it is a bit strange that imbalance_pct

> is used in two different ways to bias comparison in the same function.


I see imbalance_pct like the definition of the acceptable imbalance %
for a sched_domain. This % is then used against the current load or to
define an absolute value.

> It used to be only used as a scaling factor (now imbalance_scale), while

> this patch proposes to use it for computing an absolute margin

> (imbalance) as well. It is not major issue, but it is not clear why it

> is used differently to compare two metrics that are relatively closely

> related.


In fact, scaling factor (imbalance) doesn't work well with small
value. As an example, the use of a scaling factor fails as soon as
this_runnable_load = 0 because we always selected local rq even if
min_runnable_load is only 1  which doesn't really make sense because
they are just the same.

>

> Morten
Vincent Guittot Nov. 30, 2016, 1:54 p.m. | #3
On 30 November 2016 at 14:49, Vincent Guittot
<vincent.guittot@linaro.org> wrote:
> On 30 November 2016 at 13:49, Morten Rasmussen <morten.rasmussen@arm.com> wrote:

>> On Fri, Nov 25, 2016 at 04:34:33PM +0100, Vincent Guittot wrote:

>>> find_idlest_group() only compares the runnable_load_avg when looking for

>>> the least loaded group. But on fork intensive use case like hackbench

>

> [snip]

>

>>> +                             min_avg_load = avg_load;

>>> +                             idlest = group;

>>> +                     } else if ((runnable_load < (min_runnable_load + imbalance)) &&

>>> +                                     (100*min_avg_load > imbalance_scale*avg_load)) {

>>> +                             /*

>>> +                              * The runnable loads are close so we take

>>> +                              * into account blocked load through avg_load

>>> +                              *  which is blocked + runnable load

>>> +                              */

>>> +                             min_avg_load = avg_load;

>>>                               idlest = group;

>>>                       }

>>>

>>> @@ -5470,13 +5495,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>>>               goto no_spare;

>>>

>>>       if (this_spare > task_util(p) / 2 &&

>>> -         imbalance*this_spare > 100*most_spare)

>>> +         imbalance_scale*this_spare > 100*most_spare)

>>>               return NULL;

>>>       else if (most_spare > task_util(p) / 2)

>>>               return most_spare_sg;

>>>

>>>  no_spare:

>>> -     if (!idlest || 100*this_load < imbalance*min_load)

>>> +     if (!idlest ||

>>> +         (min_runnable_load > (this_runnable_load + imbalance)) ||

>>> +         ((this_runnable_load < (min_runnable_load + imbalance)) &&

>>> +                     (100*min_avg_load > imbalance_scale*this_avg_load)))

>>

>> I don't get why you have imbalance_scale applied to this_avg_load and

>> not min_avg_load. IIUC, you end up preferring non-local groups?

>

> In fact, I have keep the same condition that is used when looping the group.

> You're right that we should prefer local rq if avg_load are close and

> test the condition

> (100*this_avg_load > imbalance_scale*min_avg_load) instead


Of course the correct condition is
 (100*this_avg_load < imbalance_scale*min_avg_load)

>

>>

>> If we take the example where this_runnable_load == min_runnable_load and

>> this_avg_load == min_avg_load. In this case, and in cases where

>> min_avg_load is slightly bigger than this_avg_load, we end up picking

>> the 'idlest' group even if the local group is equally good or even

>> slightly better?

>>

>>>               return NULL;

>>>       return idlest;

>>>  }

>>

>> Overall, I like that load_avg is being brought in to make better

>> decisions. The variable naming is a bit confusing. For example,

>> runnable_load is a capacity-average just like avg_load. 'imbalance' is

>> now an absolute capacity-average margin, but it is hard to come up with

>> better short alternatives.

>>

>> Although 'imbalance' is based on the existing imbalance_pct, I find

>> somewhat arbitrary. Why is (imbalance_pct-100)*1024/100 a good absolute

>> margin to define the interval where we want to consider load_avg? I

>> guess it is case of 'we had to pick some value', which we have done in

>> many other places. Though, IMHO, it is a bit strange that imbalance_pct

>> is used in two different ways to bias comparison in the same function.

>

> I see imbalance_pct like the definition of the acceptable imbalance %

> for a sched_domain. This % is then used against the current load or to

> define an absolute value.

>

>> It used to be only used as a scaling factor (now imbalance_scale), while

>> this patch proposes to use it for computing an absolute margin

>> (imbalance) as well. It is not major issue, but it is not clear why it

>> is used differently to compare two metrics that are relatively closely

>> related.

>

> In fact, scaling factor (imbalance) doesn't work well with small

> value. As an example, the use of a scaling factor fails as soon as

> this_runnable_load = 0 because we always selected local rq even if

> min_runnable_load is only 1  which doesn't really make sense because

> they are just the same.

>

>>

>> Morten
Morten Rasmussen Nov. 30, 2016, 2:23 p.m. | #4
On Wed, Nov 30, 2016 at 02:49:11PM +0100, Vincent Guittot wrote:
> On 30 November 2016 at 13:49, Morten Rasmussen <morten.rasmussen@arm.com> wrote:

> > On Fri, Nov 25, 2016 at 04:34:33PM +0100, Vincent Guittot wrote:

> >> find_idlest_group() only compares the runnable_load_avg when looking for

> >> the least loaded group. But on fork intensive use case like hackbench

> 

> [snip]

> 

> >> +                             min_avg_load = avg_load;

> >> +                             idlest = group;

> >> +                     } else if ((runnable_load < (min_runnable_load + imbalance)) &&

> >> +                                     (100*min_avg_load > imbalance_scale*avg_load)) {

> >> +                             /*

> >> +                              * The runnable loads are close so we take

> >> +                              * into account blocked load through avg_load

> >> +                              *  which is blocked + runnable load

> >> +                              */

> >> +                             min_avg_load = avg_load;

> >>                               idlest = group;

> >>                       }

> >>

> >> @@ -5470,13 +5495,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

> >>               goto no_spare;

> >>

> >>       if (this_spare > task_util(p) / 2 &&

> >> -         imbalance*this_spare > 100*most_spare)

> >> +         imbalance_scale*this_spare > 100*most_spare)

> >>               return NULL;

> >>       else if (most_spare > task_util(p) / 2)

> >>               return most_spare_sg;

> >>

> >>  no_spare:

> >> -     if (!idlest || 100*this_load < imbalance*min_load)

> >> +     if (!idlest ||

> >> +         (min_runnable_load > (this_runnable_load + imbalance)) ||

> >> +         ((this_runnable_load < (min_runnable_load + imbalance)) &&

> >> +                     (100*min_avg_load > imbalance_scale*this_avg_load)))

> >

> > I don't get why you have imbalance_scale applied to this_avg_load and

> > not min_avg_load. IIUC, you end up preferring non-local groups?

> 

> In fact, I have keep the same condition that is used when looping the group.


The logic is inverted compared to the group loop as there you after
picking a better group. There it makes sense that you accept groups with
a slightly higher runnable_load if they have a much better avg_load.

Here you are after rejecting 'idlest' group if it isn't significantly
better than the local group, so the conditions have to be inverted.

> You're right that we should prefer local rq if avg_load are close and

> test the condition

> (100*this_avg_load > imbalance_scale*min_avg_load) instead


I would argue you should switch the inequality operator around:

	(100*this_avg_load < imbalance_scale*min_avg_load)

So it becomes true unless min_avg_load is significantly less than
this_avg_load meaning that we will ignore 'idlest' and return NULL. This
is also in line with old condition (100*this_load < imbalance*min_load).

> >

> > If we take the example where this_runnable_load == min_runnable_load and

> > this_avg_load == min_avg_load. In this case, and in cases where

> > min_avg_load is slightly bigger than this_avg_load, we end up picking

> > the 'idlest' group even if the local group is equally good or even

> > slightly better?

> >

> >>               return NULL;

> >>       return idlest;

> >>  }

> >

> > Overall, I like that load_avg is being brought in to make better

> > decisions. The variable naming is a bit confusing. For example,

> > runnable_load is a capacity-average just like avg_load. 'imbalance' is

> > now an absolute capacity-average margin, but it is hard to come up with

> > better short alternatives.

> >

> > Although 'imbalance' is based on the existing imbalance_pct, I find

> > somewhat arbitrary. Why is (imbalance_pct-100)*1024/100 a good absolute

> > margin to define the interval where we want to consider load_avg? I

> > guess it is case of 'we had to pick some value', which we have done in

> > many other places. Though, IMHO, it is a bit strange that imbalance_pct

> > is used in two different ways to bias comparison in the same function.

> 

> I see imbalance_pct like the definition of the acceptable imbalance %

> for a sched_domain. This % is then used against the current load or to

> define an absolute value.

> 

> > It used to be only used as a scaling factor (now imbalance_scale), while

> > this patch proposes to use it for computing an absolute margin

> > (imbalance) as well. It is not major issue, but it is not clear why it

> > is used differently to compare two metrics that are relatively closely

> > related.

> 

> In fact, scaling factor (imbalance) doesn't work well with small

> value. As an example, the use of a scaling factor fails as soon as

> this_runnable_load = 0 because we always selected local rq even if

> min_runnable_load is only 1  which doesn't really make sense because

> they are just the same.


Agreed, using the operator for scaling is not ideal for low load
scenarios. The spare-capacity checking is fixing that issue for some
scenarios but not all.

Could we mention these points somewhere so people can be reminded later?
In the commit message, if not as a comment in the code?
Morten Rasmussen Nov. 30, 2016, 2:24 p.m. | #5
On Wed, Nov 30, 2016 at 02:54:00PM +0100, Vincent Guittot wrote:
> On 30 November 2016 at 14:49, Vincent Guittot

> <vincent.guittot@linaro.org> wrote:

> > On 30 November 2016 at 13:49, Morten Rasmussen <morten.rasmussen@arm.com> wrote:

> >> On Fri, Nov 25, 2016 at 04:34:33PM +0100, Vincent Guittot wrote:

> >>> find_idlest_group() only compares the runnable_load_avg when looking for

> >>> the least loaded group. But on fork intensive use case like hackbench

> >

> > [snip]

> >

> >>> +                             min_avg_load = avg_load;

> >>> +                             idlest = group;

> >>> +                     } else if ((runnable_load < (min_runnable_load + imbalance)) &&

> >>> +                                     (100*min_avg_load > imbalance_scale*avg_load)) {

> >>> +                             /*

> >>> +                              * The runnable loads are close so we take

> >>> +                              * into account blocked load through avg_load

> >>> +                              *  which is blocked + runnable load

> >>> +                              */

> >>> +                             min_avg_load = avg_load;

> >>>                               idlest = group;

> >>>                       }

> >>>

> >>> @@ -5470,13 +5495,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

> >>>               goto no_spare;

> >>>

> >>>       if (this_spare > task_util(p) / 2 &&

> >>> -         imbalance*this_spare > 100*most_spare)

> >>> +         imbalance_scale*this_spare > 100*most_spare)

> >>>               return NULL;

> >>>       else if (most_spare > task_util(p) / 2)

> >>>               return most_spare_sg;

> >>>

> >>>  no_spare:

> >>> -     if (!idlest || 100*this_load < imbalance*min_load)

> >>> +     if (!idlest ||

> >>> +         (min_runnable_load > (this_runnable_load + imbalance)) ||

> >>> +         ((this_runnable_load < (min_runnable_load + imbalance)) &&

> >>> +                     (100*min_avg_load > imbalance_scale*this_avg_load)))

> >>

> >> I don't get why you have imbalance_scale applied to this_avg_load and

> >> not min_avg_load. IIUC, you end up preferring non-local groups?

> >

> > In fact, I have keep the same condition that is used when looping the group.

> > You're right that we should prefer local rq if avg_load are close and

> > test the condition

> > (100*this_avg_load > imbalance_scale*min_avg_load) instead

> 

> Of course the correct condition is

>  (100*this_avg_load < imbalance_scale*min_avg_load)


Agreed, I should have read the entire thread before replying :-)
Vincent Guittot Dec. 2, 2016, 3:20 p.m. | #6
On 30 November 2016 at 15:24, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> On Wed, Nov 30, 2016 at 02:54:00PM +0100, Vincent Guittot wrote:

>> On 30 November 2016 at 14:49, Vincent Guittot

>> <vincent.guittot@linaro.org> wrote:

>> > On 30 November 2016 at 13:49, Morten Rasmussen <morten.rasmussen@arm.com> wrote:

>> >> On Fri, Nov 25, 2016 at 04:34:33PM +0100, Vincent Guittot wrote:

>> >>> find_idlest_group() only compares the runnable_load_avg when looking for

>> >>> the least loaded group. But on fork intensive use case like hackbench

>> >

>> > [snip]

>> >

>> >>> +                             min_avg_load = avg_load;

>> >>> +                             idlest = group;

>> >>> +                     } else if ((runnable_load < (min_runnable_load + imbalance)) &&

>> >>> +                                     (100*min_avg_load > imbalance_scale*avg_load)) {

>> >>> +                             /*

>> >>> +                              * The runnable loads are close so we take

>> >>> +                              * into account blocked load through avg_load

>> >>> +                              *  which is blocked + runnable load

>> >>> +                              */

>> >>> +                             min_avg_load = avg_load;

>> >>>                               idlest = group;

>> >>>                       }

>> >>>

>> >>> @@ -5470,13 +5495,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,

>> >>>               goto no_spare;

>> >>>

>> >>>       if (this_spare > task_util(p) / 2 &&

>> >>> -         imbalance*this_spare > 100*most_spare)

>> >>> +         imbalance_scale*this_spare > 100*most_spare)

>> >>>               return NULL;

>> >>>       else if (most_spare > task_util(p) / 2)

>> >>>               return most_spare_sg;

>> >>>

>> >>>  no_spare:

>> >>> -     if (!idlest || 100*this_load < imbalance*min_load)

>> >>> +     if (!idlest ||

>> >>> +         (min_runnable_load > (this_runnable_load + imbalance)) ||

>> >>> +         ((this_runnable_load < (min_runnable_load + imbalance)) &&

>> >>> +                     (100*min_avg_load > imbalance_scale*this_avg_load)))

>> >>

>> >> I don't get why you have imbalance_scale applied to this_avg_load and

>> >> not min_avg_load. IIUC, you end up preferring non-local groups?

>> >

>> > In fact, I have keep the same condition that is used when looping the group.

>> > You're right that we should prefer local rq if avg_load are close and

>> > test the condition

>> > (100*this_avg_load > imbalance_scale*min_avg_load) instead

>>

>> Of course the correct condition is

>>  (100*this_avg_load < imbalance_scale*min_avg_load)

>

> Agreed, I should have read the entire thread before replying :-)


Interestingly, the original condition (100*min_avg_load >
imbalance_scale*this_avg_load) gives better performance result for the
hackbench test than the new one : (100*this_avg_load <
imbalance_scale*min_avg_load)

Matt,

Have you been able to get some results for the patchset ?

Vincent
Matt Fleming Dec. 2, 2016, 10:24 p.m. | #7
On Fri, 02 Dec, at 04:20:54PM, Vincent Guittot wrote:
> 

> Matt,

> 

> Have you been able to get some results for the patchset ?


Sorry, I messed up the test config and the tests never ran :(

I've redone everything and fast-tracked the results on the SUSE grid
now. Should have results by morning.
Brendan Gregg Dec. 3, 2016, 3:31 a.m. | #8
On Fri, Nov 25, 2016 at 7:34 AM, Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>

> find_idlest_group() only compares the runnable_load_avg when looking for

> the least loaded group. But on fork intensive use case like hackbench

> where tasks blocked quickly after the fork, this can lead to selecting the

> same CPU instead of other CPUs, which have similar runnable load but a

> lower load_avg.

>

> When the runnable_load_avg of 2 CPUs are close, we now take into account

> the amount of blocked load as a 2nd selection factor. There is now 3 zones

> for the runnable_load of the rq:

> -[0 .. (runnable_load - imbalance)] : Select the new rq which has

> significantly less runnable_load

> -](runnable_load - imbalance) .. (runnable_load + imbalance)[ : The

> runnable load are close so we use load_avg to chose between the 2 rq

> -[(runnable_load + imbalance) .. ULONG_MAX] : Keep the current rq which

> has significantly less runnable_load

>


For background, is this from the "A decade of wasted cores" paper's
patches? What's the expected typical gain? Thanks,

Brendan
Matt Fleming Dec. 3, 2016, 9:47 p.m. | #9
On Fri, 02 Dec, at 07:31:04PM, Brendan Gregg wrote:
> 

> For background, is this from the "A decade of wasted cores" paper's

> patches?


No, this patch fixes an issue I originally reported here,

  https://lkml.kernel.org/r/20160923115808.2330-1-matt@codeblueprint.co.uk

Essentially, if you have an idle or partially-idle system and a
workload that consists of fork()'ing a bunch of tasks, where each of
those tasks immediately sleeps waiting for some wakeup, then those
tasks aren't spread across all idle CPUs very well.

We saw this issue when running hackbench with a small loop count, such
that the actual benchmark setup (fork()'ing) is where the majority of
the runtime is spent.

In that scenario, there's a large potential/blocked load, but
essentially no runnable load, and the balance on fork scheduler code
only cares about runnable load without Vincent's patch applied.

The closest thing I can find in the "A decade of wasted cores" paper
is "The Overload-on-Wakeup bug", but I don't think that's the issue
here since,

  a) We're balancing on fork, not wakeup
  b) The fork on balance code balances across nodes OK

> What's the expected typical gain? Thanks,


The results are still coming back from the SUSE performance test grid
but they do show that this patch is mainly a win for multi-socket
machines with more than 8 cores or thereabouts.

 [ Vincent, I'll follow up to your PATCH 1/2 with the results that are
   specifically for that patch ]

Assuming a fork-intensive or fork-dominated workload, and a
multi-socket machine, such as this 2 socket, NUMA, with 12 cores and
HT enabled (48 cpus), we saw a very clear win between +10% and +15%
for processes communicating via pipes,

  (1) tip-sched = tip/sched/core branch
  (2) fix-fig-for-fork = (1) + PATCH 1/2
  (3) fix-sig = (1) + (2) + PATCH 2/2

hackbench-process-pipes
                         4.9.0-rc6             4.9.0-rc6             4.9.0-rc6
                         tip-sched      fix-fig-for-fork               fix-sig
Amean    1        0.0717 (  0.00%)      0.0696 (  2.99%)      0.0730 ( -1.79%)
Amean    4        0.1244 (  0.00%)      0.1200 (  3.56%)      0.1190 (  4.36%)
Amean    7        0.1891 (  0.00%)      0.1937 ( -2.42%)      0.1831 (  3.17%)
Amean    12       0.2964 (  0.00%)      0.3116 ( -5.11%)      0.2784 (  6.07%)
Amean    21       0.4011 (  0.00%)      0.4090 ( -1.96%)      0.3574 ( 10.90%)
Amean    30       0.4944 (  0.00%)      0.4654 (  5.87%)      0.4171 ( 15.63%)
Amean    48       0.6113 (  0.00%)      0.6309 ( -3.20%)      0.5331 ( 12.78%)
Amean    79       0.8616 (  0.00%)      0.8706 ( -1.04%)      0.7710 ( 10.51%)
Amean    110      1.1304 (  0.00%)      1.2211 ( -8.02%)      1.0163 ( 10.10%)
Amean    141      1.3754 (  0.00%)      1.4279 ( -3.81%)      1.2803 (  6.92%)
Amean    172      1.6217 (  0.00%)      1.7367 ( -7.09%)      1.5363 (  5.27%)
Amean    192      1.7809 (  0.00%)      2.0199 (-13.42%)      1.7129 (  3.82%)

Things look even better when using threads and pipes, with wins
between 11% and 29% when looking at results outside of the noise,

hackbench-thread-pipes
                         4.9.0-rc6             4.9.0-rc6             4.9.0-rc6
                         tip-sched      fix-fig-for-fork               fix-sig
Amean    1        0.0736 (  0.00%)      0.0794 ( -7.96%)      0.0779 ( -5.83%)
Amean    4        0.1709 (  0.00%)      0.1690 (  1.09%)      0.1663 (  2.68%)
Amean    7        0.2836 (  0.00%)      0.3080 ( -8.61%)      0.2640 (  6.90%)
Amean    12       0.4393 (  0.00%)      0.4843 (-10.24%)      0.4090 (  6.89%)
Amean    21       0.5821 (  0.00%)      0.6369 ( -9.40%)      0.5126 ( 11.95%)
Amean    30       0.6557 (  0.00%)      0.6459 (  1.50%)      0.5711 ( 12.90%)
Amean    48       0.7924 (  0.00%)      0.7760 (  2.07%)      0.6286 ( 20.68%)
Amean    79       1.0534 (  0.00%)      1.0551 ( -0.16%)      0.8481 ( 19.49%)
Amean    110      1.5286 (  0.00%)      1.4504 (  5.11%)      1.1121 ( 27.24%)
Amean    141      1.9507 (  0.00%)      1.7790 (  8.80%)      1.3804 ( 29.23%)
Amean    172      2.2261 (  0.00%)      2.3330 ( -4.80%)      1.6336 ( 26.62%)
Amean    192      2.3753 (  0.00%)      2.3307 (  1.88%)      1.8246 ( 23.19%)

Somewhat surprisingly, I can see improvements for UMA machines with
fewer cores when the workload heavily saturates the machine and the
workload isn't dominated by fork. Such heavy saturation isn't super
realistic, but still interesting. I haven't dug into why these results
occurred, but I am happy things didn't instead fall off a cliff.

Here's a 4-cpu UMA box showing some improvement at the higher end,

hackbench-process-pipes
                        4.9.0-rc6             4.9.0-rc6             4.9.0-rc6
                        tip-sched      fix-fig-for-fork               fix-sig
Amean    1       3.5060 (  0.00%)      3.5747 ( -1.96%)      3.5117 ( -0.16%)
Amean    3       7.7113 (  0.00%)      7.8160 ( -1.36%)      7.7747 ( -0.82%)
Amean    5      11.4453 (  0.00%)     11.5710 ( -1.10%)     11.3870 (  0.51%)
Amean    7      15.3147 (  0.00%)     15.9420 ( -4.10%)     15.8450 ( -3.46%)
Amean    12     25.5110 (  0.00%)     24.3410 (  4.59%)     22.6717 ( 11.13%)
Amean    16     32.3010 (  0.00%)     28.5897 ( 11.49%)     25.7473 ( 20.29%)

Patch

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 820a787..ecb5ee8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5395,16 +5395,20 @@  find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 {
 	struct sched_group *idlest = NULL, *group = sd->groups;
 	struct sched_group *most_spare_sg = NULL;
-	unsigned long min_load = ULONG_MAX, this_load = 0;
+	unsigned long min_runnable_load = ULONG_MAX, this_runnable_load = 0;
+	unsigned long min_avg_load = ULONG_MAX, this_avg_load = 0;
 	unsigned long most_spare = 0, this_spare = 0;
 	int load_idx = sd->forkexec_idx;
-	int imbalance = 100 + (sd->imbalance_pct-100)/2;
+	int imbalance_scale = 100 + (sd->imbalance_pct-100)/2;
+	unsigned long imbalance = scale_load_down(NICE_0_LOAD) *
+				(sd->imbalance_pct-100) / 100;
 
 	if (sd_flag & SD_BALANCE_WAKE)
 		load_idx = sd->wake_idx;
 
 	do {
-		unsigned long load, avg_load, spare_cap, max_spare_cap;
+		unsigned long load, avg_load, runnable_load;
+		unsigned long spare_cap, max_spare_cap;
 		int local_group;
 		int i;
 
@@ -5421,6 +5425,7 @@  find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 		 * the group containing the CPU with most spare capacity.
 		 */
 		avg_load = 0;
+		runnable_load = 0;
 		max_spare_cap = 0;
 
 		for_each_cpu(i, sched_group_cpus(group)) {
@@ -5430,7 +5435,9 @@  find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 			else
 				load = target_load(i, load_idx);
 
-			avg_load += load;
+			runnable_load += load;
+
+			avg_load += cfs_rq_load_avg(&cpu_rq(i)->cfs);
 
 			spare_cap = capacity_spare_wake(i, p);
 
@@ -5439,14 +5446,32 @@  find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 		}
 
 		/* Adjust by relative CPU capacity of the group */
-		avg_load = (avg_load * SCHED_CAPACITY_SCALE) / group->sgc->capacity;
+		avg_load = (avg_load * SCHED_CAPACITY_SCALE) /
+					group->sgc->capacity;
+		runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) /
+					group->sgc->capacity;
 
 		if (local_group) {
-			this_load = avg_load;
+			this_runnable_load = runnable_load;
+			this_avg_load = avg_load;
 			this_spare = max_spare_cap;
 		} else {
-			if (avg_load < min_load) {
-				min_load = avg_load;
+			if (min_runnable_load > (runnable_load + imbalance)) {
+				/*
+				 * The runnable load is significantly smaller
+				 *  so we can pick this new cpu
+				 */
+				min_runnable_load = runnable_load;
+				min_avg_load = avg_load;
+				idlest = group;
+			} else if ((runnable_load < (min_runnable_load + imbalance)) &&
+					(100*min_avg_load > imbalance_scale*avg_load)) {
+				/*
+				 * The runnable loads are close so we take
+				 * into account blocked load through avg_load
+				 *  which is blocked + runnable load
+				 */
+				min_avg_load = avg_load;
 				idlest = group;
 			}
 
@@ -5470,13 +5495,16 @@  find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 		goto no_spare;
 
 	if (this_spare > task_util(p) / 2 &&
-	    imbalance*this_spare > 100*most_spare)
+	    imbalance_scale*this_spare > 100*most_spare)
 		return NULL;
 	else if (most_spare > task_util(p) / 2)
 		return most_spare_sg;
 
 no_spare:
-	if (!idlest || 100*this_load < imbalance*min_load)
+	if (!idlest ||
+	    (min_runnable_load > (this_runnable_load + imbalance)) ||
+	    ((this_runnable_load < (min_runnable_load + imbalance)) &&
+			(100*min_avg_load > imbalance_scale*this_avg_load)))
 		return NULL;
 	return idlest;
 }