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Subject[PATCH RT 09/23] lib/smp_processor_id: Don't use cpumask_equal()
Date
From: Waiman Long <longman@redhat.com>

v4.14.170-rt75-rc2 stable review patch.
If anyone has any objections, please let me know.

-----------


[ 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: Tom Zanussi <zanussi@kernel.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 6f4a4ae881c8..9f3c8bb62e57 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -23,7 +23,7 @@ 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;

/*
--
2.14.1
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