Re: [PATCH v4 1/3] sched/fair: Move checking for nohz cpus after time check

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 1/13/26 2:37 PM, Vincent Guittot wrote:
> On Mon, 12 Jan 2026 at 06:05, Shrikanth Hegde <sshegde@linux.ibm.com> wrote:
>>
>> NOHZ idle load balancer is kicked off only after time check. So move
>> the atomic read after the time check to access it only when needed.
>>
>> When there are no idle CPUs(100% busy), even if the flag gets set to
>> NOHZ_STATS_KICK | NOHZ_NEXT_KICK, find_new_ilb will fail and
>> there will be no NOHZ idle balance. The current behaviour is retained.
>>
>> Note: This patch doesn't solve any cacheline overheads. No improvement
>> in performance apart from saving a few cycles of atomic_read.
> 
> But won't these cycles be then wasted by calling needlessly kick_ilb
> 

when there are nohz cpus, i.e nohz.nr_cpus > 0, there is no change in codeflow.

Only when system is 100%(which is expected to be rare), nohz.nr_cpus == 0,
then it is expected that has_blocked_load = 0. So flags shouldn't be set.
Note we are still doing a return if nohz.nr_cpus == 0. So kick_ilb shouldn't be
called.

Do you see any path still calling kick_ilb un-necessarily?


>>
>> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
>> ---
>>   kernel/sched/fair.c | 18 +++++++++++-------
>>   1 file changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 9743fc0b225c..17e4e8ac5fca 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -12451,20 +12451,24 @@ 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;
>>
>> +       /*
>> +        * If none are in tickless mode, though flag maybe set,
>> +        * idle load balancing is not done as find_new_ilb fails
>> +        */
>>          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
>>