linux/lib/smp_processor_id.c
Waiman Long e950cca3f3 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@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Acked-by:  Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191003203608.21881-1-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2019-10-09 12:46:10 +02:00

68 lines
1.4 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* lib/smp_processor_id.c
*
* DEBUG_PREEMPT variant of smp_processor_id().
*/
#include <linux/export.h>
#include <linux/kprobes.h>
#include <linux/sched.h>
notrace static nokprobe_inline
unsigned int check_preemption_disabled(const char *what1, const char *what2)
{
int this_cpu = raw_smp_processor_id();
if (likely(preempt_count()))
goto out;
if (irqs_disabled())
goto out;
/*
* Kernel threads bound to a single CPU can safely use
* smp_processor_id():
*/
if (current->nr_cpus_allowed == 1)
goto out;
/*
* It is valid to assume CPU-locality during early bootup:
*/
if (system_state < SYSTEM_SCHEDULING)
goto out;
/*
* Avoid recursion:
*/
preempt_disable_notrace();
if (!printk_ratelimit())
goto out_enable;
printk(KERN_ERR "BUG: using %s%s() in preemptible [%08x] code: %s/%d\n",
what1, what2, preempt_count() - 1, current->comm, current->pid);
printk("caller is %pS\n", __builtin_return_address(0));
dump_stack();
out_enable:
preempt_enable_no_resched_notrace();
out:
return this_cpu;
}
notrace unsigned int debug_smp_processor_id(void)
{
return check_preemption_disabled("smp_processor_id", "");
}
EXPORT_SYMBOL(debug_smp_processor_id);
NOKPROBE_SYMBOL(debug_smp_processor_id);
notrace void __this_cpu_preempt_check(const char *op)
{
check_preemption_disabled("__this_cpu_", op);
}
EXPORT_SYMBOL(__this_cpu_preempt_check);
NOKPROBE_SYMBOL(__this_cpu_preempt_check);