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authorWaiman Long <Waiman.Long@hpe.com>2016-12-14 15:04:10 -0800
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2018-05-26 08:48:54 +0200
commiteae31bc183db0281c51e09a2830d70ca20e22562 (patch)
tree490fa4097acaa82a72dab5195489886a3a084f15
parent5dd02a8952118538308b3bf5170d5db52d3ed477 (diff)
signals: avoid unnecessary taking of sighand->siglock
commit c7be96af89d4b53211862d8599b2430e8900ed92 upstream. 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> Signed-off-by: Mel Gorman <mgorman@suse.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--include/linux/signal.h17
-rw-r--r--kernel/signal.c7
2 files changed, 24 insertions, 0 deletions
diff --git a/include/linux/signal.h b/include/linux/signal.h
index d80259afb9e5..bcc094cb697c 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 4a548c6a4118..7d75bc2d042f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2495,6 +2495,13 @@ 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);