From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754586AbcI0M1j (ORCPT ); Tue, 27 Sep 2016 08:27:39 -0400 Received: from g2t2353.austin.hpe.com ([15.233.44.26]:56761 "EHLO g2t2353.austin.hpe.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751939AbcI0M1a (ORCPT ); Tue, 27 Sep 2016 08:27:30 -0400 From: Waiman Long To: Andrew Morton , Ingo Molnar , Oleg Nesterov , Thomas Gleixner , Stas Sergeev Cc: linux-kernel@vger.kernel.org, Scott J Norton , Douglas Hatch , Waiman Long Subject: [PATCH v3] signals: Avoid unnecessary taking of sighand->siglock Date: Tue, 27 Sep 2016 08:26:49 -0400 Message-Id: <1474979209-11867-1-git-send-email-Waiman.Long@hpe.com> X-Mailer: git-send-email 1.7.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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