[tip: timers/nohz] tick/nohz: Evaluate the CPU expression after the static key

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the timers/nohz branch of tip:

Commit-ID:     f105dfec0a951cd0d5bfbfe9dc067ea69f71ad5c
Gitweb:        https://git.kernel.org/tip/f105dfec0a951cd0d5bfbfe9dc067ea69f71ad5c
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 13 May 2021 01:29:15 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 13 May 2021 14:21:20 +02:00

tick/nohz: Evaluate the CPU expression after the static key

When tick_nohz_full_cpu() is called with smp_processor_id(), the latter
is unconditionally evaluated whether the static key is on or off. It is
not necessary in the off-case though, so make sure the cpu expression
is executed at the last moment.

Illustrate with the following test function:

	int tick_nohz_test(void)
	{
		return tick_nohz_full_cpu(smp_processor_id());
	}

The resulting code before was:

	mov    %gs:0x7eea92d1(%rip),%eax   # smp_processor_id() fetch
	nopl   0x0(%rax,%rax,1)
	xor    %eax,%eax
	retq
	cmpb   $0x0,0x29d393a(%rip)        # <tick_nohz_full_running>
	je     tick_nohz_test+0x29         # jump to below eax clear
	mov    %eax,%eax
	bt     %rax,0x29d3936(%rip)        # <tick_nohz_full_mask>
	setb   %al
	movzbl %al,%eax
	retq
	xor    %eax,%eax
	retq

Now it becomes:

	nopl   0x0(%rax,%rax,1)
	xor    %eax,%eax
	retq
	cmpb   $0x0,0x29d3871(%rip)        # <tick_nohz_full_running>
	je     tick_nohz_test+0x29         # jump to below eax clear
	mov    %gs:0x7eea91f0(%rip),%eax   # smp_processor_id() fetch, after static key
	mov    %eax,%eax
	bt     %rax,0x29d3866(%rip)        # <tick_nohz_full_mask>
	setb   %al
	movzbl %al,%eax
	retq
	xor    %eax,%eax
	retq

Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210512232924.150322-2-frederic@xxxxxxxxxx
---
 include/linux/tick.h | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 7340613..2258984 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -185,13 +185,17 @@ static inline bool tick_nohz_full_enabled(void)
 	return tick_nohz_full_running;
 }
 
-static inline bool tick_nohz_full_cpu(int cpu)
-{
-	if (!tick_nohz_full_enabled())
-		return false;
-
-	return cpumask_test_cpu(cpu, tick_nohz_full_mask);
-}
+/*
+ * Check if a CPU is part of the nohz_full subset. Arrange for evaluating
+ * the cpu expression (typically smp_processor_id()) _after_ the static
+ * key.
+ */
+#define tick_nohz_full_cpu(_cpu) ({					\
+	bool __ret = false;						\
+	if (tick_nohz_full_enabled())					\
+		__ret = cpumask_test_cpu((_cpu), tick_nohz_full_mask);	\
+	__ret;								\
+})
 
 static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
 {