Re: [PATCH v2 2/3] sched/fair: Change likelyhood of nohz.nr_cpus and do stats update if its due

[Shrikanth Hegde]

On 1/5/26 9:22 AM, K Prateek Nayak wrote:
> Hello Shrikanth,
>
> On 1/2/2026 6:17 PM, Shrikanth Hegde wrote:
> > These days most of the system have multi cores. The likelyhood of
> > at least one or more CPUs in nohz (idle state) is higher.
> > So move likely to unlikely.
> >
> > Allow stats balancing to complete when there are no nr_cpus as the check
> > happens later. This may do an additional stats based load balancing
> > which would reset has_blocked_load. Code also looks saner by removing
> > that uncharactiristic return in between.
> >
> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index cd1c78d2c272..5ceb9126d441 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -12456,10 +12456,10 @@ static void nohz_balancer_kick(struct rq *rq)
> >   
> >   	/*
> >   	 * None are in tickless mode and hence no need for NOHZ idle load
> > -	 * balancing:
> > +	 * balancing, do stats update if its due
> >   	 */
> > -	if (likely(!atomic_read(&nohz.nr_cpus)))
> > -		return;
> > +	if (unlikely(!atomic_read(&nohz.nohz_balancer_kick)))
> > +		goto out;

Did something got edited here?

> Since we are sure that "nohz.nr_cpus" is 0, there is a good chance
> find_new_ilb() in kick_ilb() will not find any CPU to run balance on, so
> why not just retain that return?
>
> The "flags" can only be set to (NOHZ_NEXT_KICK | NOHZ_STATS_KICK) on
> this path and kick_ilb() will simply return early without updating
> "nohz.next_balance" when it doesn't see NOHZ_BALANCE_KICK and fails to
> find any CPU. Might as well keep the early return.

The only reason why flags would be set is, if nohz.has_blocked_load
is set and time is after next_blocked. In that case, doing a stats
based balance will make nohz.has_blocked_load=0 and subsequent invocations
flags =0 and no load balance will happen if nr_cpus stays 0.

However, if we just, has_blocked_load might remains stale value.

Isn't that the case?