Messages in this thread | | | From | Barry Song <> | Date | Wed, 24 Nov 2021 13:07:36 +1300 | Subject | Re: [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu |
| |
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
| |