[PATCH v3] signals: Avoid unnecessary taking of sighand->siglock

[Waiman Long]

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.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
 v2->v3:
   - Add a sigequalsets() helper in signal.h and use it for comparison.

 v1->v2:
   - Fix compiler warning in mips.

 include/linux/signal.h |   17 +++++++++++++++++
 kernel/signal.c        |    7 +++++++
 2 files changed, 24 insertions(+), 0 deletions(-)

diff --git a/include/linux/signal.h b/include/linux/signal.h
index b63f63e..5308304 100644
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -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
diff --git a/kernel/signal.c b/kernel/signal.c
index af21afc..04e8f50 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2485,6 +2485,13 @@ 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);
-- 
1.7.1