Re: [PATCH v4 1/3] sched/fair: Move checking for nohz cpus after time check

[Shrikanth Hegde]

On 1/13/26 2:37 PM, Vincent Guittot wrote:
> On Mon, 12 Jan 2026 at 06:05, Shrikanth Hegde <sshegde@xxxxxxxxxxxxx> wrote:
> > NOHZ idle load balancer is kicked off only after time check. So move
> > the atomic read after the time check to access it only when needed.
> >
> > When there are no idle CPUs(100% busy), even if the flag gets set to
> > NOHZ_STATS_KICK | NOHZ_NEXT_KICK, find_new_ilb will fail and
> > there will be no NOHZ idle balance. The current behaviour is retained.
> >
> > Note: This patch doesn't solve any cacheline overheads. No improvement
> > in performance apart from saving a few cycles of atomic_read.
>
> But won't these cycles be then wasted by calling needlessly kick_ilb

when there are nohz cpus, i.e nohz.nr_cpus > 0, there is no change in codeflow.

Only when system is 100%(which is expected to be rare), nohz.nr_cpus == 0,
then it is expected that has_blocked_load = 0. So flags shouldn't be set.
Note we are still doing a return if nohz.nr_cpus == 0. So kick_ilb shouldn't be
called.

Do you see any path still calling kick_ilb un-necessarily?


> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 18 +++++++++++-------
> >   1 file changed, 11 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 9743fc0b225c..17e4e8ac5fca 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -12451,20 +12451,24 @@ 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;
> >
> > +       /*
> > +        * If none are in tickless mode, though flag maybe set,
> > +        * idle load balancing is not done as find_new_ilb fails
> > +        */
> >          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