From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933643AbcLNQRP (ORCPT ); Wed, 14 Dec 2016 11:17:15 -0500 Received: from mail-wm0-f65.google.com ([74.125.82.65]:35805 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754898AbcLNQQf (ORCPT ); Wed, 14 Dec 2016 11:16:35 -0500 Date: Wed, 14 Dec 2016 17:15:41 +0100 From: Michal Hocko To: "Paul E. McKenney" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, peterz@infradead.org Subject: Re: Fw: [lkp-developer] [sched,rcu] cf7a2dca60: [No primary change] +186% will-it-scale.time.involuntary_context_switches Message-ID: <20161214161540.GP25573@dhcp22.suse.cz> References: <20161213151408.GC3924@linux.vnet.ibm.com> <20161214095425.GE25573@dhcp22.suse.cz> <20161214110609.GK3924@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161214110609.GK3924@linux.vnet.ibm.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 14-12-16 03:06:09, Paul E. McKenney wrote: > On Wed, Dec 14, 2016 at 10:54:25AM +0100, Michal Hocko wrote: > > On Tue 13-12-16 07:14:08, Paul E. McKenney wrote: > > > Just FYI for the moment... > > > > > > So even with the slowed-down checking, making cond_resched() do what > > > cond_resched_rcu_qs() does results in a smallish but quite measurable > > > degradation according to 0day. > > > > So if I understand those results properly, the reason seems to be the > > increased involuntary context switches, right? Or am I misreading the > > data? > > I am looking at your "sched,rcu: Make cond_resched() provide RCU > > quiescent state" in linux-next and I am wondering whether rcu_all_qs has > > to be called unconditionally and not only when should_resched failed few > > times? I guess you have discussed that with Peter already but do not > > remember the outcome. > > My first thought is to wait for the grace period to age further before > checking, the idea being to avoid increasing cond_resched() overhead > any further. But if that doesn't work, then yes, I may have to look at > adding more checks to cond_resched(). This might be really naive but would something like the following work? The overhead should be pretty much negligible, I guess. Ideally the pcp variable could be set somewhere from check_cpu_stall() but I couldn't wrap my head around that code to see how exactly. --- diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h index ac81e4063b40..1c005c5304a3 100644 --- a/include/linux/rcutiny.h +++ b/include/linux/rcutiny.h @@ -243,6 +243,10 @@ static inline void rcu_all_qs(void) barrier(); /* Avoid RCU read-side critical sections leaking across. */ } +static inline void cond_resched_rcu_check(void) +{ +} + /* RCUtree hotplug events */ #define rcutree_prepare_cpu NULL #define rcutree_online_cpu NULL diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index 63a4e4cf40a5..176f6e386379 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -110,6 +110,18 @@ extern int rcu_scheduler_active __read_mostly; bool rcu_is_watching(void); void rcu_all_qs(void); +#ifndef CONFIG_PREEMPT +DECLARE_PER_CPU(int, rcu_needs_qs); + +static inline void cond_resched_rcu_check(void) +{ + /* Make sure we do not miss rcu_all_qs at least every now and then */ + if (this_cpu_inc_return(rcu_needs_qs) > 10) { + this_cpu_write(rcu_needs_qs, 0); + rcu_all_qs(); + } +} +#endif /* RCUtree hotplug events */ int rcutree_prepare_cpu(unsigned int cpu); diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 69a5611a7e7c..783c74ae9930 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -268,6 +268,9 @@ void rcu_bh_qs(void) } static DEFINE_PER_CPU(int, rcu_sched_qs_mask); +#ifndef CONFIG_PREEMPT +DEFINE_PER_CPU(int, rcu_needs_qs); +#endif static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = { .dynticks_nesting = DYNTICK_TASK_EXIT_IDLE, diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 154fd689fe02..a58844be2ef1 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4905,6 +4905,8 @@ int __sched _cond_resched(void) if (should_resched(0)) { preempt_schedule_common(); return 1; + } else { + cond_resched_rcu_check(); } return 0; } -- Michal Hocko SUSE Labs