Re: [PATCH] sched: Skip useless sched_balance_running acquisition if load balance is not due

[Shrikanth Hegde]

On 4/16/25 11:58, Chen, Yu C wrote:
> Hi Shrikanth,
>
> On 4/16/2025 1:30 PM, Shrikanth Hegde wrote:
> > On 4/16/25 09:28, Tim Chen wrote:
> > > At load balance time, balance of last level cache domains and
> > > above needs to be serialized. The scheduler checks the atomic var
> > > sched_balance_running first and then see if time is due for a load
> > > balance. This is an expensive operation as multiple CPUs can attempt
> > > sched_balance_running acquisition at the same time.
> > >
> > > On a 2 socket Granite Rapid systems enabling sub-numa cluster and
> > > running OLTP workloads, 7.6% of cpu cycles are spent on cmpxchg of
> > > sched_balance_running.  Most of the time, a balance attempt is aborted
> > > immediately after acquiring sched_balance_running as load balance time
> > > is not due.
> > >
> > > Instead, check balance due time first before acquiring
> > > sched_balance_running. This skips many useless acquisitions
> > > of sched_balance_running and knocks the 7.6% CPU overhead on
> > > sched_balance_domain() down to 0.05%.  Throughput of the OLTP workload
> > > improved by 11%.
> >
> > Hi Tim.
> >
> > Time check makes sense specially on large systems mainly due to 
> > NEWIDLE balance.

scratch the NEWLY_IDLE part from that comment.

> >
>
> Could you elaborate a little on this statement? There is no timeout 
> mechanism like periodic load balancer for the NEWLY_IDLE, right?

Yes. NEWLY_IDLE is very opportunistic.

> > One more point to add, A lot of time, the CPU which acquired 
> > sched_balance_running,
> > need not end up doing the load balance, since it not the CPU meant to 
> > do the load balance.
> >
> > This thread.
> > https://lore.kernel.org/all/1e43e783-55e7-417f- 
> > a1a7-503229eb163a@xxxxxxxxxxxxx/
> >
> >
> > Best thing probably is to acquire it if this CPU has passed the time 
> > check and as well it is
> > actually going to do load balance.
>
> This is a good point, and we might only want to deal with periodic load
> balancer rather than NEWLY_IDLE balance. Because the latter is too 
> frequent and contention on the sched_balance_running might introduce
> high cache contention.

But NEWLY_IDLE doesn't serialize using sched_balance_running and can 
endup consuming a lot of cycles. But if we serialize using 
sched_balance_running, it would definitely cause a lot contention as is.


The point was, before acquiring it, it would be better if this CPU is 
definite to do the load balance. Else there are chances to miss the 
actual load balance.


> thanks,
> Chenyu
>
> > > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > > Reported-by: Mohini Narkhede <mohini.narkhede@xxxxxxxxx>
> > > Tested-by: Mohini Narkhede <mohini.narkhede@xxxxxxxxx>
> > > ---
> > >   kernel/sched/fair.c | 16 ++++++++--------
> > >   1 file changed, 8 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index e43993a4e580..5e5f7a770b2f 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -12220,13 +12220,13 @@ static void sched_balance_domains(struct rq 
> > > *rq, enum cpu_idle_type idle)
> > >           interval = get_sd_balance_interval(sd, busy);
> > > -        need_serialize = sd->flags & SD_SERIALIZE;
> > > -        if (need_serialize) {
> > > -            if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
> > > -                goto out;
> > > -        }
> > > -
> > >           if (time_after_eq(jiffies, sd->last_balance + interval)) {
> > > +            need_serialize = sd->flags & SD_SERIALIZE;
> > > +            if (need_serialize) {
> > > +                if (atomic_cmpxchg_acquire(&sched_balance_running, 
> > > 0, 1))
> > > +                    goto out;
> > > +            }
> > > +
> > >               if (sched_balance_rq(cpu, rq, sd, idle, 
> > > &continue_balancing)) {
> > >                   /*
> > >                    * The LBF_DST_PINNED logic could have changed
> > > @@ -12238,9 +12238,9 @@ static void sched_balance_domains(struct rq 
> > > *rq, enum cpu_idle_type idle)
> > >               }
> > >               sd->last_balance = jiffies;
> > >               interval = get_sd_balance_interval(sd, busy);
> > > +            if (need_serialize)
> > > +                atomic_set_release(&sched_balance_running, 0);
> > >           }
> > > -        if (need_serialize)
> > > -            atomic_set_release(&sched_balance_running, 0);
> > >   out:
> > >           if (time_after(next_balance, sd->last_balance + interval)) {
> > >               next_balance = sd->last_balance + interval;