linux-kernel - [tip: locking/core] lib/smp_processor_id: Don't use cpumask

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Message-ID: <157062598194.9978.10224113529074728326.tip-bot2@tip-bot2>
Date:   Wed, 09 Oct 2019 12:59:41 -0000
From:   "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Waiman Long <longman@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] lib/smp_processor_id: Don't use cpumask_equal()

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     e950cca3f3c40902a052a78a36b3fac1f8a62d19
Gitweb:        https://git.kernel.org/tip/e950cca3f3c40902a052a78a36b3fac1f8a62d19
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Thu, 03 Oct 2019 16:36:08 -04:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Wed, 09 Oct 2019 12:46:10 +02:00

lib/smp_processor_id: Don't use cpumask_equal()

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@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Juri Lelli <juri.lelli@...hat.com>
Acked-by:  Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: https://lkml.kernel.org/r/20191003203608.21881-1-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.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 60ba93f..bd95716 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -23,7 +23,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
 	 * 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;
 
 	/*