Re: [PATCH v3 3/4] sched/fair: Add sched_smt_active check for fastpaths

[Shrikanth Hegde]

Hi Valentin. Thanks for going through the patches.

On 5/15/26 7:05 PM, Valentin Schneider wrote:
> On 13/05/26 19:09, Shrikanth Hegde wrote:
> > For fastpaths such as wakeup and load balance even minimal code additions
> > can add up. is_core_idle is accessed during load balance.
> >
> > Other callsites of is_core_idle make sched_smt_active() check first.
> > Make the same check in should_we_balance.
> >
> > Rest of access to cpu_smt_mask isn't in fastpath.
> >
> > Note: Remove the stale comment above is_core_idle. Enqueue methods
> > of fair aren't close to it anymore.
> >
> > Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> >   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 353e31ecaadc..964014a74cf9 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1578,10 +1578,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >        se->exec_start = rq_clock_task(rq_of(cfs_rq));
> >   }
> >
> > -/**************************************************
> > - * Scheduling class queueing methods:
> > - */
> > -
> > +/* Check sched_smt_active before calling this to avoid overheads in fastpaths */
> >   static inline bool is_core_idle(int cpu)
> >   {
> >        int sibling;
> > @@ -11995,7 +11992,8 @@ static int should_we_balance(struct lb_env *env)
> >                 * balancing cores, but remember the first idle SMT CPU for
> >                 * later consideration.  Find CPU on an idle core first.
> >                 */
> > -		if (!(env->sd->flags & SD_SHARE_CPUCAPACITY) && !is_core_idle(cpu)) {
> > +		if (!(env->sd->flags & SD_SHARE_CPUCAPACITY) &&
> > +		    sched_smt_active() && !is_core_idle(cpu)) {
>
> Nit: Since that whole if statement is about SMT, you could have the static
> key be the first thing checked in the condition so it can short circuit
> earlier.

Ok. We can do that too.

Only reason i had kept it that way was to avoid
calling it when load balancing at SMT domain assuming sched_smt_active() is
a more common in the field.

I can send out v4 out quickly with below change.

               if (sched_smt_active() &&
                   !(env->sd->flags & SD_SHARE_CPUCAPACITY) &&
                   !is_core_idle(cpu)) {

That's exactly what prateek had suggested.