linux-kernel - Re: [PATCH v3] sched/fair: Skip sched_balance

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Message-ID: <df068896-82f9-458d-8fff-5a2f654e8ffd@amd.com>
Date: Mon, 10 Nov 2025 09:28:42 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, Tim Chen
	<tim.c.chen@...ux.intel.com>, Peter Zijlstra <peterz@...radead.org>
CC: Ingo Molnar <mingo@...nel.org>, Chen Yu <yu.c.chen@...el.com>, Doug Nelson
	<doug.nelson@...el.com>, Mohini Narkhede <mohini.narkhede@...el.com>,
	<linux-kernel@...r.kernel.org>, Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v3] sched/fair: Skip sched_balance_running cmpxchg when
 balance is not due

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.

-- 
Thanks and Regards,
Prateek