linux-kernel - [PATCH v2] timer: Use static_branch_likely() for timers_nohz

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Message-ID: <20210624163218.39ae3ec6@xhacker.debian>
Date:   Thu, 24 Jun 2021 16:32:18 +0800
From:   Jisheng Zhang <Jisheng.Zhang@...aptics.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH v2] timer: Use static_branch_likely() for timers_nohz_active

The static key timers_nohz_active is likely to be true, so use
static_branch_likely() to reflect this fact.

Per include/linux/jump_label.h:

 * type\branch|	likely (1)	      |	unlikely (0)
 * -----------+-----------------------+------------------
 *            |                       |
 *  true (1)  |	   ...		      |	   ...
 *            |    NOP		      |	   JMP L
 *            |    <br-stmts>	      |	1: ...
 *            |	L: ...		      |
 *            |			      |
 *            |			      |	L: <br-stmts>
 *            |			      |	   jmp 1b
 *            |                       |
 * -----------+-----------------------+------------------

This could improve the finally generated code a bit for the most likely
scenario, I.E save two "jmp" instructions.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@...aptics.com>
---
Since v1:
 - update the commit msg to avoid confusing and add some descriptions about
   where's the improvement from.

 kernel/time/timer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index d111adf4a0cb..40d4fe7fa207 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -265,7 +265,7 @@ int timer_migration_handler(struct ctl_table *table, int write,
 
 static inline bool is_timers_nohz_active(void)
 {
-	return static_branch_unlikely(&timers_nohz_active);
+	return static_branch_likely(&timers_nohz_active);
 }
 #else
 static inline bool is_timers_nohz_active(void) { return false; }
-- 
2.31.0