linux-kernel - Re: [PATCH v2 1/3] sched/fair: Move checking for nohz cpus after time check

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Message-ID: <a17a790d-31e2-4843-aa95-577caaa88afb@linux.ibm.com>
Date: Tue, 6 Jan 2026 21:27:46 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        kprateek.nayak@....com, juri.lelli@...hat.com, vschneid@...hat.com,
        tglx@...utronix.de, dietmar.eggemann@....com, anna-maria@...utronix.de,
        frederic@...nel.org, wangyang.guo@...el.com
Subject: Re: [PATCH v2 1/3] sched/fair: Move checking for nohz cpus after time
 check


Hi Peter.

On 1/6/26 9:20 PM, Peter Zijlstra wrote:
> On Fri, Jan 02, 2026 at 06:17:42PM +0530, Shrikanth Hegde wrote:
>> Idle load balancer is kicked off only after time check. So move
>> the atomic read after the time check to avoid the overhead.
> 
> Presumably this is because the nr_cpus field is more contended than the
> next_balance field; but aren't they in the same cacheline? That is,
> *what* !??!

Main reason for cacheline contention is nr_cpus in nohz struct.
Other one is idle_cpus_mask, but that is in different
cacheline itself(most of the time)

This patch is not due to next_balance being contended. It is
simply to check the value when we are sure time has passed to
attempt NOHZ balance again.

> 
>> If there are no nohz CPUs and next_blocked has passed, then there
>> will be one additional stats based load balancing which would set the
>> has_blocked_load to 0. It shouldn't make a difference.
>>
>> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
>> ---
>>   kernel/sched/fair.c | 14 +++++++-------
>>   1 file changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 7377f9117501..cd1c78d2c272 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -12447,13 +12447,6 @@ static void nohz_balancer_kick(struct rq *rq)
>>   	 */
>>   	nohz_balance_exit_idle(rq);
>>   
>> -	/*
>> -	 * None are in tickless mode and hence no need for NOHZ idle load
>> -	 * balancing:
>> -	 */
>> -	if (likely(!atomic_read(&nohz.nr_cpus)))
>> -		return;
>> -
>>   	if (READ_ONCE(nohz.has_blocked_load) &&
>>   	    time_after(now, READ_ONCE(nohz.next_blocked)))
>>   		flags = NOHZ_STATS_KICK;
>> @@ -12461,6 +12454,13 @@ static void nohz_balancer_kick(struct rq *rq)
>>   	if (time_before(now, nohz.next_balance))
>>   		goto out;
>>   
>> +	/*
>> +	 * None are in tickless mode and hence no need for NOHZ idle load
>> +	 * balancing:
>> +	 */
>> +	if (likely(!atomic_read(&nohz.nr_cpus)))
>> +		return;
>> +
>>   	if (rq->nr_running >= 2) {
>>   		flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>>   		goto out;
>> -- 
>> 2.47.3
>>