On Fri, Jul 24, 2026 at 07:37:26PM +0530, Shrikanth Hegde wrote:
> When cpu is marked as non preferred, any load pulled towards it is
> pointless since in the next tick task will be pushed out again.
> So, Consider only preferred CPUs for load balance.
>
> This makes it not fight against the push task mechanism which happens
> at tick. Also, this stops active balance to happen on non-preferred CPU
> pulling the load.
>
> This means there is no load balancing if the task is pinned only to
> non-preferred CPUs. They will continue to run where they were previously
> running before the CPUs was marked as non-preferred.
>
> Bailout early for NEWIDLE and IDLE balance as load balancing is done
> only on preferred CPUs.
>
> Signed-off-by: Shrikanth Hegde <[email protected]>
> ---
> kernel/sched/fair.c | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index df8c9c2c7918..12f5b7de28d2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13399,7 +13399,7 @@ static int sched_balance_rq(int this_cpu, struct rq
> *this_rq,
> };
> bool need_unlock = false;
>
> - cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
> + cpumask_and(cpus, sched_domain_span(sd), cpu_preferred_mask);
>
> schedstat_inc(sd->lb_count[idle]);
>
> @@ -14337,7 +14337,8 @@ static void _nohz_idle_balance(struct rq *this_rq,
> unsigned int flags)
> update_rq_clock(rq);
> rq_unlock_irqrestore(rq, &rf);
>
> - if (flags & NOHZ_BALANCE_KICK)
> + if (flags & NOHZ_BALANCE_KICK &&
> + cpu_preferred(balance_cpu))
> sched_balance_domains(rq, CPU_IDLE);
Here you skip the sched_balance_domains() for idle non-preferred CPUs,
which means you don't re-calculate the rq->next_balance.
In the following code, we update the global nohz.next_balance
depending on the new rq->next_balance, which doesn't happen if
the sched_balance_domains() is not invoked.
So, you propagate an already-expired per-CPU deadline back into the
global NOHZ deadline. It may lead to unneeded asynchronous IPI in the
nohz_balancer_kick() -> kick_ilb() path.
I think, you need another helper, something like:
void sched_balance_domains(rq, idle)
{
__sched_balance_domains(rq, idle);
advance_rq_next_balance(rq, idle);
}
And then in the code above:
if (flags & NOHZ_BALANCE_KICK) {
if (cpu_preferred(balance_cpu))
__sched_balance_domains(rq, CPU_IDLE);
advance_rq_next_balance(rq, idle);
}
> @@ -14481,10 +14482,8 @@ static int sched_balance_newidle(struct rq *this_rq,
> struct rq_flags *rf)
> */
> this_rq->idle_stamp = rq_clock(this_rq);
>
> - /*
> - * Do not pull tasks towards !active CPUs...
> - */
> - if (!cpu_active(this_cpu))
> + /* Do not pull tasks towards !preferred CPUs */
> + if (!cpu_preferred(this_cpu))
> return 0;
>
> /*
> --
> 2.47.3
