linux-kernel - Re: [PATCH v2 01/12] sched/psi: Optimize psi_group_change() cpu

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Message-ID: <20250716104050.GR1613200@noisy.programming.kicks-ass.net>
Date: Wed, 16 Jul 2025 12:40:50 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Beata Michalska <beata.michalska@....com>
Cc: Chris Mason <clm@...a.com>, mingo@...hat.com, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, linux-kernel@...r.kernel.org,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v2 01/12] sched/psi: Optimize psi_group_change()
 cpu_clock() usage

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@...radead.org>
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@...a.com>
Suggested-by: Beata Michalska <beata.michalska@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 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);