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

From: Qi Zheng
Date: Tue Aug 11 2020 - 07:44:46 EST

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
2. The following inequality has already been checked in
group_is_overloaded() which was also called in

(sgs->group_capacity * imbalance_pct) <
(sgs->group_runnable * 100)

Consider group_is_overloaded() returns false because of the first

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cpumask are no longer active at the == case,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?