From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1034355AbcIVSZp (ORCPT ); Thu, 22 Sep 2016 14:25:45 -0400 Received: from g2t2355.austin.hpe.com ([15.233.44.28]:59898 "EHLO g2t2355.austin.hpe.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1033311AbcIVSZm (ORCPT ); Thu, 22 Sep 2016 14:25:42 -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] signals: Avoid unnecessary taking of sighand->siglock Date: Thu, 22 Sep 2016 14:25:05 -0400 Message-Id: <1474568705-40114-1-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. This patch adds a check in 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. This patch is currently only active for 64-bit architectures. Signed-off-by: Waiman Long --- kernel/signal.c | 10 ++++++++++ 1 files changed, 10 insertions(+), 0 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index af21afc..5850b11 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2485,6 +2485,16 @@ void __set_current_blocked(const sigset_t *newset) { struct task_struct *tsk = current; + /* + * In case the signal mask hasn't changed, we won't need to take + * the lock. As the current blocked mask can be modified by other + * CPUs, we need to do an atomic read without lock. In other words, + * this check will only be done on 64-bit systems. + */ +#if _NSIG_WORDS == 1 + if (READ_ONCE(tsk->blocked.sig[0]) == newset->sig[0]) + return; +#endif spin_lock_irq(&tsk->sighand->siglock); __set_task_blocked(tsk, newset); spin_unlock_irq(&tsk->sighand->siglock); -- 1.7.1