From 3df3c9149f18b2e3acaee0522e78d1c101a936ca Mon Sep 17 00:00:00 2001
Message-Id: <3df3c9149f18b2e3acaee0522e78d1c101a936ca.1594742967.git.zanussi@kernel.org>
In-Reply-To: <832f7d97d6b989a5b4860dd2dbec58ad6ad5ab81.1594742966.git.zanussi@kernel.org>
References: <832f7d97d6b989a5b4860dd2dbec58ad6ad5ab81.1594742966.git.zanussi@kernel.org>
From: Waiman Long <longman@redhat.com>
Date: Thu, 3 Oct 2019 16:36:08 -0400
Subject: [PATCH 302/329] lib/smp_processor_id: Don't use cpumask_equal()
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.132-rt59.tar.xz

[ 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;
 
 	/*
-- 
2.17.1

