From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760976AbcIWQ44 (ORCPT ); Fri, 23 Sep 2016 12:56:56 -0400 Received: from g9t1613g.houston.hpe.com ([15.241.32.99]:10955 "EHLO g9t1613g.houston.hpe.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752923AbcIWQ4y (ORCPT ); Fri, 23 Sep 2016 12:56:54 -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 v2] signals: Avoid unnecessary taking of sighand->siglock Date: Fri, 23 Sep 2016 12:56:14 -0400 Message-Id: <1474649774-48834-1-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. This patch is currently only active for 64-bit architectures. Signed-off-by: Waiman Long --- v1->v2: - Fix compiler warning in mips. kernel/signal.c | 10 ++++++++++ 1 files changed, 10 insertions(+), 0 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index af21afc..e4296b6 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. The current blocked mask can be modified by other CPUs. + * To be safe, we need to do an atomic read without lock. As a result, + * this check will only be done on 64-bit architectures. + */ + if ((_NSIG_WORDS == 1) && + (READ_ONCE(tsk->blocked.sig[0]) == newset->sig[0])) + return; + spin_lock_irq(&tsk->sighand->siglock); __set_task_blocked(tsk, newset); spin_unlock_irq(&tsk->sighand->siglock); -- 1.7.1