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)
{
|