Re: [PATCH] sched/pelt: Fix truncation of the PELT period count

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Hui,

On 10/22/2026 9:19 AM, Hui Su wrote:
> accumulate_sum() computes the number of elapsed PELT periods in a
> u64 but returns u32. When the count is a multiple of 2^32, it is

So at least an hour between PELT updates! I suppose it is possible
with nohz so not too far fetched.

> truncated to zero even though the PELT sums have been updated.
> 
> The caller interprets zero as no period boundary crossed and skips
> recomputing the averages, leaving *_avg stale relative to *_sum.
> 
> Return only whether any period elapsed, which is all the caller
> needs.
> 

Probably needs a:

Fixes: a481db34b9be ("sched/fair: Optimize ___update_sched_avg()") # c079629862b2

> Signed-off-by: Hui Su <sh_def@163.com>
> ---
>  kernel/sched/pelt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index 779eb58a4261..519bc9c40043 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -146,7 +146,7 @@ accumulate_sum(u64 delta, struct sched_avg *sa,
>  	if (running)
>  		sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;
>  
> -	return periods;
> +	return !!periods;

I would actually favor changing return type to u64. Since
accumulate_sum() is only called in one place, and that too within an
'if' condition, I'm sure compiler can optimize it similarly.

>  }
>  
>  /*

-- 
Thanks and Regards,
Prateek