Re: [PATCH v2 1/3] sched/fair: Move checking for nohz cpus after time check

[Shrikanth Hegde]

Hi Peter.

On 1/6/26 9:20 PM, Peter Zijlstra wrote:
> On Fri, Jan 02, 2026 at 06:17:42PM +0530, Shrikanth Hegde wrote:
> > Idle load balancer is kicked off only after time check. So move
> > the atomic read after the time check to avoid the overhead.
>
> Presumably this is because the nr_cpus field is more contended than the
> next_balance field; but aren't they in the same cacheline? That is,
> *what* !??!

Main reason for cacheline contention is nr_cpus in nohz struct.
Other one is idle_cpus_mask, but that is in different
cacheline itself(most of the time)

This patch is not due to next_balance being contended. It is
simply to check the value when we are sure time has passed to
attempt NOHZ balance again.

> > If there are no nohz CPUs and next_blocked has passed, then there
> > will be one additional stats based load balancing which would set the
> > has_blocked_load to 0. It shouldn't make a difference.
> >
> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 14 +++++++-------
> >   1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 7377f9117501..cd1c78d2c272 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -12447,13 +12447,6 @@ static void nohz_balancer_kick(struct rq *rq)
> >   	 */
> >   	nohz_balance_exit_idle(rq);
> >   
> > -	/*
> > -	 * None are in tickless mode and hence no need for NOHZ idle load
> > -	 * balancing:
> > -	 */
> > -	if (likely(!atomic_read(&nohz.nr_cpus)))
> > -		return;
> > -
> >   	if (READ_ONCE(nohz.has_blocked_load) &&
> >   	    time_after(now, READ_ONCE(nohz.next_blocked)))
> >   		flags = NOHZ_STATS_KICK;
> > @@ -12461,6 +12454,13 @@ static void nohz_balancer_kick(struct rq *rq)
> >   	if (time_before(now, nohz.next_balance))
> >   		goto out;
> >   
> > +	/*
> > +	 * None are in tickless mode and hence no need for NOHZ idle load
> > +	 * balancing:
> > +	 */
> > +	if (likely(!atomic_read(&nohz.nr_cpus)))
> > +		return;
> > +
> >   	if (rq->nr_running >= 2) {
> >   		flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
> >   		goto out;
> > --
> > 2.47.3