Re: [PATCH v2 01/12] sched/psi: Optimize psi_group_change() cpu_clock() usage

[Aithal, Srikanth]

On 7/16/2025 4:10 PM, Peter Zijlstra wrote:
> On Wed, Jul 16, 2025 at 08:53:01AM +0200, Beata Michalska wrote:
> > Wouldn't it be enough to use SEQCNT_ZERO? Those are static per-cpu ones.
>
> Yeah, I suppose that should work. The below builds, but I've not yet
> observed the issue myself.
>
> ---
> Subject: sched/psi: Fix psi_seq initialization
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Tue, 15 Jul 2025 15:11:14 -0400
>
> With the seqcount moved out of the group into a global psi_seq,
> re-initializing the seqcount on group creation is causing seqcount
> corruption.
>
> Fixes: 570c8efd5eb7 ("sched/psi: Optimize psi_group_change() cpu_clock() usage")
> Reported-by: Chris Mason <clm@xxxxxxxx>
> Suggested-by: Beata Michalska <beata.michalska@xxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>   kernel/sched/psi.c |    6 +-----
>   1 file changed, 1 insertion(+), 5 deletions(-)
>
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -176,7 +176,7 @@ struct psi_group psi_system = {
>   	.pcpu = &system_group_pcpu,
>   };
>   
> -static DEFINE_PER_CPU(seqcount_t, psi_seq);
> +static DEFINE_PER_CPU(seqcount_t, psi_seq) = SEQCNT_ZERO(psi_seq);
>   
>   static inline void psi_write_begin(int cpu)
>   {
> @@ -204,11 +204,7 @@ static void poll_timer_fn(struct timer_l
>   
>   static void group_init(struct psi_group *group)
>   {
> -	int cpu;
> -
>   	group->enabled = true;
> -	for_each_possible_cpu(cpu)
> -		seqcount_init(per_cpu_ptr(&psi_seq, cpu));
>   	group->avg_last_update = sched_clock();
>   	group->avg_next_update = group->avg_last_update + psi_period;
>   	mutex_init(&group->avgs_lock);

I've tested the above patch, and it resolves the issue. Could we include 
this patch in the linux-next builds? We have been encountering the 
reported issue regularly in our daily CI.


Tested-by: Srikanth Aithal <sraithal@xxxxxxx>