linux-kernel - Re: [PATCH v2 2/3] sched/fair: Change likelyhood of nohz.nr

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Message-ID: <880e3610-dbdb-42a8-9ddb-ab2e7d3cdc1f@linux.ibm.com>
Date: Mon, 5 Jan 2026 10:37:27 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: linux-kernel@...r.kernel.org, juri.lelli@...hat.com, vschneid@...hat.com,
        tglx@...utronix.de, dietmar.eggemann@....com, anna-maria@...utronix.de,
        frederic@...nel.org, wangyang.guo@...el.com, mingo@...nel.org,
        peterz@...radead.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v2 2/3] sched/fair: Change likelyhood of nohz.nr_cpus and
 do stats update if its due



On 1/5/26 9:22 AM, K Prateek Nayak wrote:
> Hello Shrikanth,
> 
> On 1/2/2026 6:17 PM, Shrikanth Hegde wrote:
>> These days most of the system have multi cores. The likelyhood of
>> at least one or more CPUs in nohz (idle state) is higher.
>> So move likely to unlikely.
>>
>> Allow stats balancing to complete when there are no nr_cpus as the check
>> happens later. This may do an additional stats based load balancing
>> which would reset has_blocked_load. Code also looks saner by removing
>> that uncharactiristic return in between.
>>
>> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
>> ---
>>   kernel/sched/fair.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index cd1c78d2c272..5ceb9126d441 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -12456,10 +12456,10 @@ static void nohz_balancer_kick(struct rq *rq)
>>   
>>   	/*
>>   	 * None are in tickless mode and hence no need for NOHZ idle load
>> -	 * balancing:
>> +	 * balancing, do stats update if its due
>>   	 */
>> -	if (likely(!atomic_read(&nohz.nr_cpus)))
>> -		return;
>> +	if (unlikely(!atomic_read(&nohz.nohz_balancer_kick)))
>> +		goto out;

Did something got edited here?

> Since we are sure that "nohz.nr_cpus" is 0, there is a good chance
> find_new_ilb() in kick_ilb() will not find any CPU to run balance on, so
> why not just retain that return?
> 
> The "flags" can only be set to (NOHZ_NEXT_KICK | NOHZ_STATS_KICK) on
> this path and kick_ilb() will simply return early without updating
> "nohz.next_balance" when it doesn't see NOHZ_BALANCE_KICK and fails to
> find any CPU. Might as well keep the early return.
> 

The only reason why flags would be set is, if nohz.has_blocked_load
is set and time is after next_blocked. In that case, doing a stats
based balance will make nohz.has_blocked_load=0 and subsequent invocations
flags =0 and no load balance will happen if nr_cpus stays 0.

However, if we just, has_blocked_load might remains stale value.

Isn't that the case?