From f105dfec0a951cd0d5bfbfe9dc067ea69f71ad5c Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 13 May 2021 01:29:15 +0200 Subject: [PATCH] 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) # je tick_nohz_test+0x29 # jump to below eax clear mov %eax,%eax bt %rax,0x29d3936(%rip) # 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) # 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) # setb %al movzbl %al,%eax retq xor %eax,%eax retq Signed-off-by: Peter Zijlstra Signed-off-by: Frederic Weisbecker Signed-off-by: Ingo Molnar Link: https://lore.kernel.org/r/20210512232924.150322-2-frederic@kernel.org --- 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 7340613c7eff..2258984a0e8a 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) { -- 2.25.1