Re: [PATCH v3] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

[Tim Chen <tim.c.chen@linux.intel.com>]

On Mon, 2025-11-10 at 09:28 +0530, K Prateek Nayak wrote:
> Hello Shrikanth,
> 
> On 11/7/2025 2:27 PM, Shrikanth Hegde wrote:
> > > >   @@ -11768,6 +11784,13 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
> > > >           goto out_balanced;
> > > >       }
> > > >   +    if (idle != CPU_NEWLY_IDLE && (sd->flags & SD_SERIALIZE)) {
> > 
> > Can you also try removing "idle != CPU_NEWLY_IDLE" and see the workload behavior?
> > If workloads don't observe regression, it might be worth serializing it too.
> 
> P.S. In one of my previous testing, I had tested only serialized for
> !env.idle (__CPU_NOT_IDLE) and I didn't spot any difference in my
> benchmark runs compared to always serializing.
> 
> I believe the "max_newidle_lb_cost" along with the plethora of
> need_resched() checks we have help bail out of newidle balance if
> there is a wakeup on the same CPU.
> 
> Idle balance too was okay with a greater number of search. If the
> first CPU of group fails to pull any task and remains idle, all
> the other idle CPUs simply bail out at should_we_balance() which
> is probably why there was no difference in the set of benchmarks I
> tested.
> 
> Serializing all shouldn't make it any worse that what we have now
> so I don't mind either.

Serializing the CPU_NEWLY_IDLE case does not make things worse
from our testing. I will be posting an updated patch shortly.

Tim