tick/nohz: Evaluate the CPU expression after the static key
authorPeter Zijlstra <peterz@infradead.org>
Wed, 12 May 2021 23:29:15 +0000 (01:29 +0200)
committerIngo Molnar <mingo@kernel.org>
Thu, 13 May 2021 12:21:20 +0000 (14:21 +0200)
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@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210512232924.150322-2-frederic@kernel.org
include/linux/tick.h

index 7340613..2258984 100644 (file)
@@ -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)
 {