[PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[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

Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[Peter Zijlstra]

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.

Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[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

Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[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

Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[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

Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[Peter Zijlstra]

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 :-)

Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[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