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From: Waiman Long <longman@redhat.com>
Date: Thu, 3 Oct 2019 16:36:08 -0400
Subject: [PATCH 301/353] 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=79718592757dc1a6c28913770376a3027354b206
[ 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 <longman@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
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;
/*
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