* [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
@ 2021-11-23 11:22 Barry Song
2021-11-23 21:07 ` Peter Zijlstra
0 siblings, 1 reply; 3+ messages in thread
From: Barry Song @ 2021-11-23 11:22 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>
---
kernel/sched/fair.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ff69f24..e2fb3e0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6298,9 +6298,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
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;
time = cpu_clock(this);
}
@@ -6312,11 +6312,11 @@ 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] 3+ messages in thread
* Re: [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-23 11:22 [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu Barry Song
@ 2021-11-23 21:07 ` Peter Zijlstra
2021-11-24 0:07 ` Barry Song
0 siblings, 1 reply; 3+ messages in thread
From: Peter Zijlstra @ 2021-11-23 21:07 UTC (permalink / raw)
To: Barry Song
Cc: mingo, juri.lelli, vincent.guittot, rostedt, linux-kernel,
dietmar.eggemann, bsegall, mgorman, bristot, Barry Song
On Tue, Nov 23, 2021 at 07:22:29PM +0800, Barry Song wrote:
> 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>
> ---
> kernel/sched/fair.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff69f24..e2fb3e0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6298,9 +6298,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>
> 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;
>
> time = cpu_clock(this);
> }
> @@ -6312,11 +6312,11 @@ 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;
> }
> }
That's just confusing code. Isn't it much clearer to write the whole
thing like so ?
nr--;
for_each_cpu_wrap(cpu, cpus, target+1) {
...
if (!nr--)
return -1;
}
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
2021-11-23 21:07 ` Peter Zijlstra
@ 2021-11-24 0:07 ` Barry Song
0 siblings, 0 replies; 3+ messages in thread
From: Barry Song @ 2021-11-24 0:07 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 Wed, Nov 24, 2021 at 10:07 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Nov 23, 2021 at 07:22:29PM +0800, Barry Song wrote:
> > 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>
> > ---
> > kernel/sched/fair.c | 8 ++++----
> > 1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index ff69f24..e2fb3e0 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6298,9 +6298,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >
> > 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;
> >
> > time = cpu_clock(this);
> > }
> > @@ -6312,11 +6312,11 @@ 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;
> > }
> > }
>
> That's just confusing code. Isn't it much clearer to write the whole
> thing like so ?
>
> nr--;
this is fine to avoid the code of setting 4 to 3 and setting
div_u64(span_avg, avg_cost) to
div_u64(span_avg, avg_cost) - 1;
> for_each_cpu_wrap(cpu, cpus, target+1) {
> ...
> if (!nr--)
I guess you mean if(!--nr).
For example, if nr=4, the original code will only check 3 cpus for
__select_idle_cpu.
the new code "nr--" will check 4 cpus for __select_idle_cpu. to keep the amount
untouched, the code should be --nr.
> return -1;
> }
>
Thanks
Barry
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2021-11-23 11:22 [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu Barry Song
2021-11-23 21:07 ` Peter Zijlstra
2021-11-24 0:07 ` Barry Song
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