On Wed, Sep 09, 2026 at 07:26:11PM +0530, Shrikanth Hegde wrote: > When a CPU is marked as non-preferred, any load pulled towards it is > pointless since the task will be pushed out again in the next tick. > So, consider only preferred CPUs for load balancing. > > This ensures load balancing does not fight against the push task mechanism > which happens at the tick. Also, this stops active balancing from happening > on a non-preferred CPU pulling the load. > > This also means there is no load balancing if a task is pinned only to > non-preferred CPUs. They will continue to run where they were previously > running before the CPUs were marked as non-preferred. > > Bail out early for NEWIDLE balancing, as load balancing is done only on > preferred CPUs. Note that idle balancing is allowed to go through, since > that naturally updates nohz.next_balance when all the idle CPUs are > non-preferred. > > Also, optimization in find_new_ilb() is skipped. The steal governor driver, > which is introduced in later patches, updates the preferred CPUs state in > descending order. find_new_ilb() checks for idle CPUs in ascending order. > Hence, in most common scenarios, the idle CPU found by find_new_ilb() will > already be a preferred CPU. When all idle CPUs are non-preferred, the first > idle CPU has to be chosen anyway. All of this is naturally handled in > find_new_ilb() currently. Adding additional complexity to it for rare > edge cases is not necessary. > > Signed-off-by: Shrikanth Hegde <[email protected]>
Reviewed-by: Yury Norov <[email protected]> > --- > kernel/sched/fair.c | 8 +++----- > 1 file changed, 3 insertions(+), 5 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index b8bd308c2d5b..4ef1167b8c73 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -13473,7 +13473,7 @@ 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]); > > @@ -14588,10 +14588,8 @@ 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; > > /* > -- > 2.52.0
