Re: [PATCH] sched/fair: Remove the duplicate check from group_has_capacity()

[Qi Zheng]

On 2020/8/11 下午6:38, Valentin Schneider wrote:
> On 11/08/20 04:39, Qi Zheng wrote:
> > On 2020/8/11 上午2:33, Valentin Schneider wrote:
> > > On 10/08/20 02:00, Qi Zheng wrote:
> > > > 1. The group_has_capacity() function is only called in
> > > >      group_classify().
> > > > 2. The following inequality has already been checked in
> > > >      group_is_overloaded() which was also called in
> > > >      group_classify().
> > > >
> > > >         (sgs->group_capacity * imbalance_pct) <
> > > >                           (sgs->group_runnable * 100)
> > >
> > > Consider group_is_overloaded() returns false because of the first
> > > condition:
> > >
> > >           if (sgs->sum_nr_running <= sgs->group_weight)
> > >                   return false;
> > >
> > > then group_has_capacity() would be the first place where the group_runnable
> > > vs group_capacity comparison would be done.
> > >
> > > Now in that specific case we'll actually only check it if
> > >
> > >     sgs->sum_nr_running == sgs->group_weight
> > >
> > > and the only case where the runnable vs capacity check can fail here is if
> > > there's significant capacity pressure going on. TBH this capacity pressure
> > > could be happening even when there are fewer tasks than CPUs, so I'm not
> > > sure how intentional that corner case is.
> >
> > Maybe some cpus in sg->cpumask are no longer active at the == case,
> > which causes the significant capacity pressure?
>
> That can only happen in that short window between deactivating a CPU and
> not having rebuilt the sched_domains yet, which sounds quite elusive.

In fact, at the beginning, I added unlikely() here to hint the compiler:

-	if ((sgs->group_capacity * imbalance_pct) <
-			(sgs->group_runnable * 100))
+	if (unlikely((sgs->group_capacity * imbalance_pct) <
+			(sgs->group_runnable * 100)))

The corresponding patch is as follows:

	[PATCH]sched/core: add unlikely in group_has_capacity()

Do you think it is necessary?