linux-kernel - Re: [PATCH v4] sched/fair: Skip sched_balance

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Message-ID: <55e02921-6477-4ed0-9ef6-16c3f34594a8@linux.ibm.com>
Date: Wed, 12 Nov 2025 16:39:43 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>, 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>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Srikar Dronamraju <srikar@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4] sched/fair: Skip sched_balance_running cmpxchg when
 balance is not due



On 11/12/25 4:15 PM, Peter Zijlstra wrote:
> On Wed, Nov 12, 2025 at 11:37:40AM +0100, Peter Zijlstra wrote:
>> On Wed, Nov 12, 2025 at 01:32:23PM +0530, Srikar Dronamraju wrote:
>>>>   	group = sched_balance_find_src_group(&env);
>>>>   	if (!group) {
>>>>   		schedstat_inc(sd->lb_nobusyg[idle]);
>>>> @@ -11892,6 +11916,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>>>>   			if (!cpumask_subset(cpus, env.dst_grpmask)) {
>>>>   				env.loop = 0;
>>>>   				env.loop_break = SCHED_NR_MIGRATE_BREAK;
>>>> +				if (need_unlock)
>>>> +					atomic_set_release(&sched_balance_running, 0);
>>>> +
>>>
>>> One nit:
>>> While the current code is good, would conditionally resetting the
>>> need_unlock just after resetting the atomic variable better than
>>> unconditional reset that we do now?
>>
>> Right, I had the same thought when grabbed the patch yesterday, but
>> ignored it.
>>
> 
> Hmm, should we not redo while keeping the lock? Doesn't make much sense
> to drop and try to reacquire things here.
> 
> So perhaps this is the better option -- or did I overlook something with
> should_we_balance? It doesn't look like that will make a different
> decision on the retry.
> 

I think in newidle balance, these checks are there in swb to bail of load balance.
redo logic catches it right?

env->dst_rq lock is taken only in attach_tasks, meanwhile, if the wakeup happened,
pending would be set. is irq enabled or remote CPU can set ttwu_pending on this rq?


         if (env->idle == CPU_NEWLY_IDLE) {
                 if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending)
                         return 0;
                 return 1;
         }