* [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
@ 2021-11-24 9:15 Barry Song
2021-11-24 11:13 ` Peter Zijlstra
0 siblings, 1 reply; 7+ messages in thread
From: Barry Song @ 2021-11-24 9:15 UTC (permalink / raw)
To: mingo, peterz, juri.lelli, vincent.guittot, rostedt, linux-kernel
Cc: dietmar.eggemann, bsegall, mgorman, bristot, Barry Song
From: Barry Song <song.bao.hua@hisilicon.com>
This patch keeps the same scanning amount, but drops a redundant loop
of cpumask_next_wrap.
The original code did for_each_cpu_wrap(cpu, cpus, target + 1), then
checked --nr; this patch does --nr before doing the next loop, thus,
it can remove a cpumask_next_wrap() which costs a little bit.
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
---
-v2: make code clearer with respect to Peter's comment
kernel/sched/fair.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e476f6..8cd23f1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6278,6 +6278,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
time = cpu_clock(this);
}
+ --nr;
for_each_cpu_wrap(cpu, cpus, target + 1) {
if (has_idle_core) {
i = select_idle_core(p, cpu, cpus, &idle_cpu);
@@ -6285,11 +6286,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
return i;
} else {
- if (!--nr)
- return -1;
idle_cpu = __select_idle_cpu(cpu, p);
if ((unsigned int)idle_cpu < nr_cpumask_bits)
break;
+ if (!--nr)
+ return -1;
}
}
--
1.8.3.1
^ permalink raw reply related [flat|nested] 7+ messages in thread
* Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-24 9:15 [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu Barry Song
@ 2021-11-24 11:13 ` Peter Zijlstra
2021-11-24 11:49 ` Barry Song
0 siblings, 1 reply; 7+ messages in thread
From: Peter Zijlstra @ 2021-11-24 11:13 UTC (permalink / raw)
To: Barry Song
Cc: mingo, juri.lelli, vincent.guittot, rostedt, linux-kernel,
dietmar.eggemann, bsegall, mgorman, bristot, Barry Song
On Wed, Nov 24, 2021 at 05:15:46PM +0800, Barry Song wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6e476f6..8cd23f1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6278,6 +6278,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> time = cpu_clock(this);
> }
>
> + --nr;
> for_each_cpu_wrap(cpu, cpus, target + 1) {
> if (has_idle_core) {
> i = select_idle_core(p, cpu, cpus, &idle_cpu);
> @@ -6285,11 +6286,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> return i;
>
> } else {
> - if (!--nr)
> - return -1;
> idle_cpu = __select_idle_cpu(cpu, p);
> if ((unsigned int)idle_cpu < nr_cpumask_bits)
> break;
> + if (!--nr)
> + return -1;
> }
> }
This way nr can never be 1 for a single iteration -- it current isn't,
but that's besides the point.
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-24 11:13 ` Peter Zijlstra
@ 2021-11-24 11:49 ` Barry Song
2021-11-24 11:57 ` Barry Song
0 siblings, 1 reply; 7+ messages in thread
From: Barry Song @ 2021-11-24 11:49 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Ingo Molnar, Juri Lelli, Vincent Guittot, Steven Rostedt, LKML,
Dietmar Eggemann, Ben Segall, Mel Gorman,
Daniel Bristot de Oliveira, Barry Song
On Thu, Nov 25, 2021 at 12:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Nov 24, 2021 at 05:15:46PM +0800, Barry Song wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 6e476f6..8cd23f1 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6278,6 +6278,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > time = cpu_clock(this);
> > }
> >
> > + --nr;
> > for_each_cpu_wrap(cpu, cpus, target + 1) {
> > if (has_idle_core) {
> > i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > @@ -6285,11 +6286,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > return i;
> >
> > } else {
> > - if (!--nr)
> > - return -1;
> > idle_cpu = __select_idle_cpu(cpu, p);
> > if ((unsigned int)idle_cpu < nr_cpumask_bits)
> > break;
> > + if (!--nr)
> > + return -1;
> > }
> > }
>
> This way nr can never be 1 for a single iteration -- it current isn't,
> but that's besides the point.
Yep. nr=1 seems to be a corner case.
if nr=1, the original code will return -1 directly without scanning
any cpu. but the new code will scan
one time because I haven't checked if(!--nr) and return before
for_each_cpu_wrap(). so this changes
the behaviour for this corner case.
but if i change "--nr" to "nr--", this new code will scan nr times
rather than nr-1, this changes the behaviour
for all cases besides nr!=1. The original code was looping nr times
but scanning nr-1 times only.
so you prefer here the codes should scan nr times and change the
scanning amount from nr-1
to nr?
Thanks
Barry
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-24 11:49 ` Barry Song
@ 2021-11-24 11:57 ` Barry Song
2021-11-24 12:02 ` Barry Song
0 siblings, 1 reply; 7+ messages in thread
From: Barry Song @ 2021-11-24 11:57 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Ingo Molnar, Juri Lelli, Vincent Guittot, Steven Rostedt, LKML,
Dietmar Eggemann, Ben Segall, Mel Gorman,
Daniel Bristot de Oliveira, Barry Song
On Thu, Nov 25, 2021 at 12:49 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Thu, Nov 25, 2021 at 12:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Nov 24, 2021 at 05:15:46PM +0800, Barry Song wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 6e476f6..8cd23f1 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6278,6 +6278,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > time = cpu_clock(this);
> > > }
> > >
> > > + --nr;
> > > for_each_cpu_wrap(cpu, cpus, target + 1) {
> > > if (has_idle_core) {
> > > i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > > @@ -6285,11 +6286,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > return i;
> > >
> > > } else {
> > > - if (!--nr)
> > > - return -1;
> > > idle_cpu = __select_idle_cpu(cpu, p);
> > > if ((unsigned int)idle_cpu < nr_cpumask_bits)
> > > break;
> > > + if (!--nr)
> > > + return -1;
> > > }
> > > }
> >
> > This way nr can never be 1 for a single iteration -- it current isn't,
> > but that's besides the point.
>
> Yep. nr=1 seems to be a corner case.
> if nr=1, the original code will return -1 directly without scanning
> any cpu. but the new code will scan
> one time because I haven't checked if(!--nr) and return before
> for_each_cpu_wrap(). so this changes
> the behaviour for this corner case.
>
> but if i change "--nr" to "nr--", this new code will scan nr times
> rather than nr-1, this changes the behaviour
> for all cases besides nr!=1. The original code was looping nr times
> but scanning nr-1 times only.
>
> so you prefer here the codes should scan nr times and change the
> scanning amount from nr-1
> to nr?
Let me make it clearer. if nr=5, the original code will loop 5 times,
but in the 5th loop, it returns directly, so __select_idle_cpu is
only done 4 times.
if nr=1, the original code will loop 1 time, but in the 1st loop,
it returns directly, so __select_idle_cpu is done 0 times.
if i change the code to if(!nr--), while nr=5, the new code will
do __select_idle_cpu() 5 times rather than 4 times in the
original code.
but of course the new code changes the __select_idle_cpu
from zero to one time for the corner case nr==1.
>
> Thanks
> Barry
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-24 11:57 ` Barry Song
@ 2021-11-24 12:02 ` Barry Song
2021-11-24 15:02 ` Peter Zijlstra
0 siblings, 1 reply; 7+ messages in thread
From: Barry Song @ 2021-11-24 12:02 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Ingo Molnar, Juri Lelli, Vincent Guittot, Steven Rostedt, LKML,
Dietmar Eggemann, Ben Segall, Mel Gorman,
Daniel Bristot de Oliveira, Barry Song
On Thu, Nov 25, 2021 at 12:57 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Thu, Nov 25, 2021 at 12:49 AM Barry Song <21cnbao@gmail.com> wrote:
> >
> > On Thu, Nov 25, 2021 at 12:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Wed, Nov 24, 2021 at 05:15:46PM +0800, Barry Song wrote:
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index 6e476f6..8cd23f1 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -6278,6 +6278,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > > time = cpu_clock(this);
> > > > }
> > > >
> > > > + --nr;
> > > > for_each_cpu_wrap(cpu, cpus, target + 1) {
> > > > if (has_idle_core) {
> > > > i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > > > @@ -6285,11 +6286,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > > return i;
> > > >
> > > > } else {
> > > > - if (!--nr)
> > > > - return -1;
> > > > idle_cpu = __select_idle_cpu(cpu, p);
> > > > if ((unsigned int)idle_cpu < nr_cpumask_bits)
> > > > break;
> > > > + if (!--nr)
> > > > + return -1;
> > > > }
> > > > }
> > >
> > > This way nr can never be 1 for a single iteration -- it current isn't,
> > > but that's besides the point.
> >
> > Yep. nr=1 seems to be a corner case.
> > if nr=1, the original code will return -1 directly without scanning
> > any cpu. but the new code will scan
> > one time because I haven't checked if(!--nr) and return before
> > for_each_cpu_wrap(). so this changes
> > the behaviour for this corner case.
> >
> > but if i change "--nr" to "nr--", this new code will scan nr times
> > rather than nr-1, this changes the behaviour
> > for all cases besides nr!=1. The original code was looping nr times
> > but scanning nr-1 times only.
> >
> > so you prefer here the codes should scan nr times and change the
> > scanning amount from nr-1
> > to nr?
>
> Let me make it clearer. if nr=5, the original code will loop 5 times,
> but in the 5th loop, it returns directly, so __select_idle_cpu is
> only done 4 times.
>
> if nr=1, the original code will loop 1 time, but in the 1st loop,
> it returns directly, so __select_idle_cpu is done 0 times.
this is also why in the first version of patch, i did this:
span_avg = sd->span_weight * avg_idle;
if (span_avg > 4*avg_cost)
- nr = div_u64(span_avg, avg_cost);
+ nr = div_u64(span_avg, avg_cost) - 1;
else
- nr = 4;
+ nr = 3;
because we are actually scanning 3 times or div_u64(span_avg, avg_cost) - 1
times but not 4 times or div_u64(span_avg, avg_cost) times.
this is not confusing at all. the only thing which is confusing is the original
code.
>
> if i change the code to if(!nr--), while nr=5, the new code will
> do __select_idle_cpu() 5 times rather than 4 times in the
> original code.
>
> but of course the new code changes the __select_idle_cpu
> from zero to one time for the corner case nr==1.
> >
> > Thanks
> > Barry
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-24 12:02 ` Barry Song
@ 2021-11-24 15:02 ` Peter Zijlstra
2021-11-24 20:49 ` Barry Song
0 siblings, 1 reply; 7+ messages in thread
From: Peter Zijlstra @ 2021-11-24 15:02 UTC (permalink / raw)
To: Barry Song
Cc: Ingo Molnar, Juri Lelli, Vincent Guittot, Steven Rostedt, LKML,
Dietmar Eggemann, Ben Segall, Mel Gorman,
Daniel Bristot de Oliveira, Barry Song
On Thu, Nov 25, 2021 at 01:02:00AM +1300, Barry Song wrote:
> On Thu, Nov 25, 2021 at 12:57 AM Barry Song <21cnbao@gmail.com> wrote:
> > Let me make it clearer. if nr=5, the original code will loop 5 times,
> > but in the 5th loop, it returns directly, so __select_idle_cpu is
> > only done 4 times.
> >
> > if nr=1, the original code will loop 1 time, but in the 1st loop,
> > it returns directly, so __select_idle_cpu is done 0 times.
>
> this is also why in the first version of patch, i did this:
> span_avg = sd->span_weight * avg_idle;
> if (span_avg > 4*avg_cost)
> - nr = div_u64(span_avg, avg_cost);
> + nr = div_u64(span_avg, avg_cost) - 1;
> else
> - nr = 4;
> + nr = 3;
>
> because we are actually scanning 3 times or div_u64(span_avg, avg_cost) - 1
> times but not 4 times or div_u64(span_avg, avg_cost) times.
It still is confusing, because > 4*span -> nr = avg/span, very much
implies we want to bottom out at 4.
> this is not confusing at all. the only thing which is confusing is the original
> code.
But yes, it seems a whole lot of confusion stacked together. Let make it
sane and say that we do 'nr' iterations, because clearly that was the
intent :-)
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-24 15:02 ` Peter Zijlstra
@ 2021-11-24 20:49 ` Barry Song
0 siblings, 0 replies; 7+ messages in thread
From: Barry Song @ 2021-11-24 20:49 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Ingo Molnar, Juri Lelli, Vincent Guittot, Steven Rostedt, LKML,
Dietmar Eggemann, Ben Segall, Mel Gorman,
Daniel Bristot de Oliveira, Barry Song
On Thu, Nov 25, 2021 at 4:02 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Nov 25, 2021 at 01:02:00AM +1300, Barry Song wrote:
> > On Thu, Nov 25, 2021 at 12:57 AM Barry Song <21cnbao@gmail.com> wrote:
>
> > > Let me make it clearer. if nr=5, the original code will loop 5 times,
> > > but in the 5th loop, it returns directly, so __select_idle_cpu is
> > > only done 4 times.
> > >
> > > if nr=1, the original code will loop 1 time, but in the 1st loop,
> > > it returns directly, so __select_idle_cpu is done 0 times.
> >
> > this is also why in the first version of patch, i did this:
> > span_avg = sd->span_weight * avg_idle;
> > if (span_avg > 4*avg_cost)
> > - nr = div_u64(span_avg, avg_cost);
> > + nr = div_u64(span_avg, avg_cost) - 1;
> > else
> > - nr = 4;
> > + nr = 3;
> >
> > because we are actually scanning 3 times or div_u64(span_avg, avg_cost) - 1
> > times but not 4 times or div_u64(span_avg, avg_cost) times.
>
> It still is confusing, because > 4*span -> nr = avg/span, very much
> implies we want to bottom out at 4.
>
> > this is not confusing at all. the only thing which is confusing is the original
> > code.
>
> But yes, it seems a whole lot of confusion stacked together. Let make it
> sane and say that we do 'nr' iterations, because clearly that was the
> intent :-)
yes. It seems this is much more sensible to do iterations in the
number of nr rather than
nr-1. we can achieve this goal by two ways:
(1)
nr--;
for_each_cpu_wrap(cpu, cpus, target + 1) {
_select_idle_cpu()....
if (!nr--)
return;
}
(2)
for_each_cpu_wrap(cpu, cpus, target + 1) {
_select_idle_cpu()....
if (!--nr)
return;
}
it seems the second way is still better as we don't need the "nr--" before
for_each_cpu_wrap() ?
Thanks
Barry
^ permalink raw reply [flat|nested] 7+ messages in thread
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2021-11-24 9:15 [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu Barry Song
2021-11-24 11:13 ` Peter Zijlstra
2021-11-24 11:49 ` Barry Song
2021-11-24 11:57 ` Barry Song
2021-11-24 12:02 ` Barry Song
2021-11-24 15:02 ` Peter Zijlstra
2021-11-24 20:49 ` Barry Song
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