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

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Message-ID: <CAKfTPtBF353mFXrqdm9_QbfhDJKsvOpjvER+p+X61XEeAd=URA@mail.gmail.com>
Date: Wed, 16 Apr 2025 11:47:22 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: "Chen, Yu C" <yu.c.chen@...el.com>, Tim Chen <tim.c.chen@...ux.intel.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, 
	Doug Nelson <doug.nelson@...el.com>, Mohini Narkhede <mohini.narkhede@...el.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Skip useless sched_balance_running acquisition if
 load balance is not due

On Wed, 16 Apr 2025 at 11:29, Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:
>
>
>
> On 4/16/25 14:46, Shrikanth Hegde wrote:
> >
> >
> > 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@...ux.ibm.com/
> >>>
> >>>
> >>> 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.
> >
> >
>
> Sorry, forgot to add.
>
> Do we really need newidle running all the way till NUMA? or if it runs till PKG is it enough?
> the regular (idle) can take care for NUMA by serializing it?
>
> -               if (sd->flags & SD_BALANCE_NEWIDLE) {
> +               if (sd->flags & SD_BALANCE_NEWIDLE && !(sd->flags & SD_SERIALIZE)) {

Why not just clearing SD_BALANCE_NEWIDLE in your sched domain when you
set SD_SERIALIZE

>
>                          pulled_task = sched_balance_rq(this_cpu, this_rq,
>                                                     sd, CPU_NEWLY_IDLE,
>
>
> Anyways, having a policy around this SD_SERIALIZE would be a good thing.
>
> >> thanks,
> >> Chenyu
> >>
> >>>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> >>>> Reported-by: Mohini Narkhede <mohini.narkhede@...el.com>
> >>>> Tested-by: Mohini Narkhede <mohini.narkhede@...el.com>
> >>>> ---
> >>>>   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;
> >>>
> >
>