linux-kernel - [PATCH] sched/core: Use try_cmpxchg in set_nr_{and_not,if}

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Message-Id: <20220519170747.56094-1-ubizjak@gmail.com>
Date:   Thu, 19 May 2022 19:07:47 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Christian Brauner <brauner@...nel.org>
Subject: [PATCH] sched/core: Use try_cmpxchg in set_nr_{and_not,if}_polling

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) != old in
set_nr_{and_not,if}_polling. x86 cmpxchg returns success in ZF flag,
so this change saves a compare after cmpxchg.

Also declare these two functions as inline. In the case of
set_nr_and_not_polling, the compiler (gcc) tries to outsmart itself
by constructing the boolean return value with logic operations on the
fetched value, and these extra operations enlarge the function over
the inlining threshold value.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Christian Brauner <brauner@...nel.org>
---
 kernel/sched/core.c | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d58c0389eb23..5b160bc85cd6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -883,15 +883,11 @@ static inline void hrtick_rq_init(struct rq *rq)
 	({								\
 		typeof(ptr) _ptr = (ptr);				\
 		typeof(mask) _mask = (mask);				\
-		typeof(*_ptr) _old, _val = *_ptr;			\
+		typeof(*_ptr) _val = *_ptr;				\
 									\
-		for (;;) {						\
-			_old = cmpxchg(_ptr, _val, _val | _mask);	\
-			if (_old == _val)				\
-				break;					\
-			_val = _old;					\
-		}							\
-	_old;								\
+		do {							\
+		} while (!try_cmpxchg(_ptr, &_val, _val | _mask));	\
+	_val;								\
 })
 
 #if defined(CONFIG_SMP) && defined(TIF_POLLING_NRFLAG)
@@ -900,7 +896,7 @@ static inline void hrtick_rq_init(struct rq *rq)
  * this avoids any races wrt polling state changes and thereby avoids
  * spurious IPIs.
  */
-static bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct task_struct *p)
 {
 	struct thread_info *ti = task_thread_info(p);
 	return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
@@ -912,33 +908,31 @@ static bool set_nr_and_not_polling(struct task_struct *p)
  * If this returns true, then the idle task promises to call
  * sched_ttwu_pending() and reschedule soon.
  */
-static bool set_nr_if_polling(struct task_struct *p)
+static inline bool set_nr_if_polling(struct task_struct *p)
 {
 	struct thread_info *ti = task_thread_info(p);
-	typeof(ti->flags) old, val = READ_ONCE(ti->flags);
+	typeof(ti->flags) val = READ_ONCE(ti->flags);
 
 	for (;;) {
 		if (!(val & _TIF_POLLING_NRFLAG))
 			return false;
 		if (val & _TIF_NEED_RESCHED)
 			return true;
-		old = cmpxchg(&ti->flags, val, val | _TIF_NEED_RESCHED);
-		if (old == val)
+		if (try_cmpxchg(&ti->flags, &val, val | _TIF_NEED_RESCHED))
 			break;
-		val = old;
 	}
 	return true;
 }
 
 #else
-static bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct task_struct *p)
 {
 	set_tsk_need_resched(p);
 	return true;
 }
 
 #ifdef CONFIG_SMP
-static bool set_nr_if_polling(struct task_struct *p)
+static inline bool set_nr_if_polling(struct task_struct *p)
 {
 	return false;
 }
-- 
2.35.3