Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754511AbbH0MxI (ORCPT ); Thu, 27 Aug 2015 08:53:08 -0400 Received: from mail-wi0-f177.google.com ([209.85.212.177]:37043 "EHLO mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752940AbbH0MxF (ORCPT ); Thu, 27 Aug 2015 08:53:05 -0400 Date: Thu, 27 Aug 2015 14:53:02 +0200 From: Frederic Weisbecker To: Jason Low Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Oleg Nesterov , "Paul E. McKenney" , linux-kernel@vger.kernel.org, Linus Torvalds , Davidlohr Bueso , Steven Rostedt , Andrew Morton , Terry Rudd , Rik van Riel , Scott J Norton Subject: Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Message-ID: <20150827125300.GA21105@lerouge> References: <1440559068-29680-1-git-send-email-jason.low2@hp.com> <1440559068-29680-4-git-send-email-jason.low2@hp.com> <20150826225628.GD11992@lerouge> <1440631954.32300.26.camel@j-VirtualBox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1440631954.32300.26.camel@j-VirtualBox> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4711 Lines: 112 On Wed, Aug 26, 2015 at 04:32:34PM -0700, Jason Low wrote: > On Thu, 2015-08-27 at 00:56 +0200, Frederic Weisbecker wrote: > > On Tue, Aug 25, 2015 at 08:17:48PM -0700, Jason Low wrote: > > > It was found while running a database workload on large systems that > > > significant time was spent trying to acquire the sighand lock. > > > > > > The issue was that whenever an itimer expired, many threads ended up > > > simultaneously trying to send the signal. Most of the time, nothing > > > happened after acquiring the sighand lock because another thread > > > had already sent the signal and updated the "next expire" time. The > > > fastpath_timer_check() didn't help much since the "next expire" time > > > was updated later. > > > > > > This patch addresses this by having the thread_group_cputimer structure > > > maintain a boolean to signify when a thread in the group is already > > > checking for process wide timers, and adds extra logic in the fastpath > > > to check the boolean. > > > > > > Signed-off-by: Jason Low > > > --- > > > include/linux/init_task.h | 1 + > > > include/linux/sched.h | 3 +++ > > > kernel/time/posix-cpu-timers.c | 19 +++++++++++++++++-- > > > 3 files changed, 21 insertions(+), 2 deletions(-) > > > > > > diff --git a/include/linux/init_task.h b/include/linux/init_task.h > > > index d0b380e..3350c77 100644 > > > --- a/include/linux/init_task.h > > > +++ b/include/linux/init_task.h > > > @@ -53,6 +53,7 @@ extern struct fs_struct init_fs; > > > .cputimer = { \ > > > .cputime_atomic = INIT_CPUTIME_ATOMIC, \ > > > .running = 0, \ > > > + .checking_timer = 0, \ > > > }, \ > > > INIT_PREV_CPUTIME(sig) \ > > > .cred_guard_mutex = \ > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > > index 119823d..a6c8334 100644 > > > --- a/include/linux/sched.h > > > +++ b/include/linux/sched.h > > > @@ -619,6 +619,8 @@ struct task_cputime_atomic { > > > * @cputime_atomic: atomic thread group interval timers. > > > * @running: non-zero when there are timers running and > > > * @cputime receives updates. > > > + * @checking_timer: non-zero when a thread is in the process of > > > + * checking for thread group timers. > > > * > > > * This structure contains the version of task_cputime, above, that is > > > * used for thread group CPU timer calculations. > > > @@ -626,6 +628,7 @@ struct task_cputime_atomic { > > > struct thread_group_cputimer { > > > struct task_cputime_atomic cputime_atomic; > > > int running; > > > + int checking_timer; > > > > How about a flag in the "running" field instead? > > > > 1) Space in signal_struct is not as important as in task_strut but it > > still matters. > > George Spelvin suggested that we convert them to booleans which would > make them take up 2 bytes. > > > 2) We already read the "running" field locklessly. Adding a new field like > > checking_timer gets even more complicated. Ideally there should be at > > least a paired memory barrier between both. Let's just simplify that > > with a single field. > > hmmm, so having 1 "flag" where we access bits for the "running" and > "checking_timer"? Sure, like: #define CPUTIMER_RUNNING 0x1 #define CPUTIMER_CHECKING 0x2 struct thread_group_cputimer { struct task_cputime_atomic cputime_atomic; int status; } So from cputimer_running() you just need to check: if (cputimer->status & CPUTIMER_RUNNING) And from run_posix_cpu_timer() fast-path: if (cputimer->status == CPUTIMER_RUNNING) so that ignores CPUTIMER_CHECKING case. > > > Now concerning the solution for your problem, I'm a bit uncomfortable with > > lockless magics like this. When the thread sets checking_timer to 1, there is > > no guarantee that the other threads in the process will see it "fast" enough > > to avoid the slow path checks. Then there is also the risk that the threads > > don't see "fast" enough that checking_timers has toggled to 0 and as a result > > a timer may expire late. Now the lockless access of "running" already induces > > such race. So if it really solves issues in practice, why not. > > Perhaps to be safer, we use something like load_acquire() and > store_release() for accessing both the ->running and ->checking_timer > fields? Well it depends against what we want to order them. If it's a single field we don't need to order them together at least. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/