linux-kernel - [PATCH] sched/cputime: Simplify seq retry logic in thread_group

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Message-ID: <20250731121506.6423-1-lirongqing@baidu.com>
Date: Thu, 31 Jul 2025 20:15:06 +0800
From: lirongqing <lirongqing@...du.com>
To: <mingo@...hat.com>, <peterz@...radead.org>, <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>
CC: Li RongQing <lirongqing@...du.com>
Subject: [PATCH] sched/cputime: Simplify seq retry logic in thread_group_cputime()

From: Li RongQing <lirongqing@...du.com>

The new logic maintains equivalent functionality but streamlines the code:
  1. First iteration: seq becomes 2 (even -> lockless path)
  2. Contended iterations: seq becomes odd -> locked path
Eliminates redundant state tracking variables

Signed-off-by: Li RongQing <lirongqing@...du.com>
---
 kernel/sched/cputime.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 7097de2..7eeff8d 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -315,7 +315,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
 	struct signal_struct *sig = tsk->signal;
 	u64 utime, stime;
 	struct task_struct *t;
-	unsigned int seq, nextseq;
+	unsigned int seq = 1;
 	unsigned long flags;
 
 	/*
@@ -330,10 +330,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
 		(void) task_sched_runtime(current);
 
 	rcu_read_lock();
-	/* Attempt a lockless read on the first round. */
-	nextseq = 0;
 	do {
-		seq = nextseq;
+		seq++; /* 2 on the 1st/lockless path, otherwise odd */
 		flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
 		times->utime = sig->utime;
 		times->stime = sig->stime;
@@ -345,8 +343,6 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
 			times->stime += stime;
 			times->sum_exec_runtime += read_sum_exec_runtime(t);
 		}
-		/* If lockless access failed, take the lock. */
-		nextseq = 1;
 	} while (need_seqretry(&sig->stats_lock, seq));
 	done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
 	rcu_read_unlock();
-- 
2.9.4