From: Waiman Long <longman@redhat.com>
To: Ingo Molnar <mingo@kernel.org>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
Masami Hiramatsu <mhiramat@kernel.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Juri Lelli <jlelli@redhat.com>, Waiman Long <longman@redhat.com>
Subject: [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal()
Date: Thu, 3 Oct 2019 16:36:08 -0400 [thread overview]
Message-ID: <20191003203608.21881-1-longman@redhat.com> (raw)
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>
---
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 60ba93fc42ce..bd9571653288 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;
/*
--
2.18.1
next reply other threads:[~2019-10-03 20:36 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-03 20:36 Waiman Long [this message]
2019-10-04 9:20 ` [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal() Sebastian Andrzej Siewior
2019-10-04 12:30 ` Waiman Long
2019-10-04 10:32 ` Juri Lelli
2019-10-07 12:39 ` Peter Zijlstra
2019-10-09 12:59 ` [tip: locking/core] " tip-bot2 for Waiman Long
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