Commit c7be96af authored Dec 14, 2016 by Waiman Long Committed by Linus Torvalds Dec 14, 2016

signals: avoid unnecessary taking of sighand->siglock

When running certain database workload on a high-end system with many
CPUs, it was found that spinlock contention in the sigprocmask syscalls
became a significant portion of the overall CPU cycles as shown below.

  9.30%  9.30%  905387  dataserver  /proc/kcore 0x7fff8163f4d2
  [k] _raw_spin_lock_irq
            |
            ---_raw_spin_lock_irq
               |
               |--99.34%-- __set_current_blocked
               |          sigprocmask
               |          sys_rt_sigprocmask
               |          system_call_fastpath
               |          |
               |          |--50.63%-- __swapcontext
               |          |          |
               |          |          |--99.91%-- upsleepgeneric
               |          |
               |          |--49.36%-- __setcontext
               |          |          ktskRun

Looking further into the swapcontext function in glibc, it was found that
the function always call sigprocmask() without checking if there are
changes in the signal mask.

A check was added to the __set_current_blocked() function to avoid taking
the sighand->siglock spinlock if there is no change in the signal mask.
This will prevent unneeded spinlock contention when many threads are
trying to call sigprocmask().

With this patch applied, the spinlock contention in sigprocmask() was
gone.

Link: http://lkml.kernel.org/r/1474979209-11867-1-git-send-email-Waiman.Long@hpe.com


Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stas Sergeev <stsp@list.ru>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 73e64c51

include/linux/signal.h

+17 −0

Original line number	Diff line number	Diff line
		@@ -97,6 +97,23 @@ static inline int sigisemptyset(sigset_t *set)
		}
		}

		static inline int sigequalsets(const sigset_t set1, const sigset_t set2)
		{
		switch (_NSIG_WORDS) {
		case 4:
		return (set1->sig[3] == set2->sig[3]) &&
		(set1->sig[2] == set2->sig[2]) &&
		(set1->sig[1] == set2->sig[1]) &&
		(set1->sig[0] == set2->sig[0]);
		case 2:
		return (set1->sig[1] == set2->sig[1]) &&
		(set1->sig[0] == set2->sig[0]);
		case 1:
		return set1->sig[0] == set2->sig[0];
		}
		return 0;
		}

		#define sigmask(sig) (1UL << ((sig) - 1))

		#ifndef __HAVE_ARCH_SIG_SETOPS

kernel/signal.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -2491,6 +2491,13 @@ void __set_current_blocked(const sigset_t *newset)
		{
		struct task_struct *tsk = current;

		/*
		* In case the signal mask hasn't changed, there is nothing we need
		* to do. The current->blocked shouldn't be modified by other task.
		*/
		if (sigequalsets(&tsk->blocked, newset))
		return;

		spin_lock_irq(&tsk->sighand->siglock);
		__set_task_blocked(tsk, newset);
		spin_unlock_irq(&tsk->sighand->siglock);

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