From: Waiman Long Date: Thu, 3 Oct 2019 16:36:08 -0400 Subject: [PATCH 301/351] lib/smp_processor_id: Don't use cpumask_equal() Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=abc76798c863ebc7979792b6f6f45bafccf1eeb3 [ Upstream commit 659252061477862f45b79e1de169e6030f5c8918 ] The check_preemption_disabled() function uses cpumask_equal() to see if the task is bounded to the current CPU only. cpumask_equal() calls memcmp() to do the comparison. As x86 doesn't have __HAVE_ARCH_MEMCMP, the slow memcmp() function in lib/string.c is used. On a RT kernel that call check_preemption_disabled() very frequently, below is the perf-record output of a certain microbenchmark: 42.75% 2.45% testpmd [kernel.kallsyms] [k] check_preemption_disabled 40.01% 39.97% testpmd [kernel.kallsyms] [k] memcmp We should avoid calling memcmp() in performance critical path. So the cpumask_equal() call is now replaced with an equivalent simpler check. Signed-off-by: Waiman Long Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Steven Rostedt (VMware) --- lib/smp_processor_id.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c index fb35c45b9421..b8a8a8db2d75 100644 --- a/lib/smp_processor_id.c +++ b/lib/smp_processor_id.c @@ -22,7 +22,7 @@ notrace static unsigned int check_preemption_disabled(const char *what1, * Kernel threads bound to a single CPU can safely use * smp_processor_id(): */ - if (cpumask_equal(current->cpus_ptr, cpumask_of(this_cpu))) + if (current->nr_cpus_allowed == 1) goto out; /*