From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754591Ab3G2Dcc (ORCPT ); Sun, 28 Jul 2013 23:32:32 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:45471 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1752676Ab3G2DcK (ORCPT ); Sun, 28 Jul 2013 23:32:10 -0400 X-IronPort-AV: E=Sophos;i="4.89,766,1367942400"; d="scan'208";a="8053015" Message-ID: <51F5E325.4000607@cn.fujitsu.com> Date: Mon, 29 Jul 2013 11:36:05 +0800 From: Lai Jiangshan User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc14 Thunderbird/3.1.4 MIME-Version: 1.0 To: "Paul E. McKenney" CC: linux-kernel@vger.kernel.org, mingo@elte.hu, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH RFC nohz_full 7/7] nohz_full: Force RCU's grace-period kthreads onto timekeeping CPU References: <20130726231848.GA12967@linux.vnet.ibm.com> <1374880764-14248-1-git-send-email-paulmck@linux.vnet.ibm.com> <1374880764-14248-7-git-send-email-paulmck@linux.vnet.ibm.com> In-Reply-To: <1374880764-14248-7-git-send-email-paulmck@linux.vnet.ibm.com> X-MIMETrack: Itemize by SMTP Server on mailserver/fnst(Release 8.5.3|September 15, 2011) at 2013/07/29 11:29:59, Serialize by Router on mailserver/fnst(Release 8.5.3|September 15, 2011) at 2013/07/29 11:30:00, Serialize complete at 2013/07/29 11:30:00 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/27/2013 07:19 AM, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > Because RCU's quiescent-state-forcing mechanism is used to drive the > full-system-idle state machine, and because this mechanism is executed > by RCU's grace-period kthreads, this commit forces these kthreads to > run on the timekeeping CPU (tick_do_timer_cpu). To do otherwise would > mean that the RCU grace-period kthreads would force the system into > non-idle state every time they drove the state machine, which would > be just a bit on the futile side. > > Signed-off-by: Paul E. McKenney > Cc: Frederic Weisbecker > Cc: Steven Rostedt > --- > kernel/rcutree.c | 1 + > kernel/rcutree.h | 1 + > kernel/rcutree_plugin.h | 20 +++++++++++++++++++- > 3 files changed, 21 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index aa6d96e..fe83085 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -1286,6 +1286,7 @@ static int rcu_gp_init(struct rcu_state *rsp) > struct rcu_data *rdp; > struct rcu_node *rnp = rcu_get_root(rsp); > > + rcu_bind_gp_kthread(); > raw_spin_lock_irq(&rnp->lock); > rsp->gp_flags = 0; /* Clear all flags: New grace period. */ bind the gp thread when RCU_GP_FLAG_INIT ... > > diff --git a/kernel/rcutree.h b/kernel/rcutree.h > index e0de5dc..49dac99 100644 > --- a/kernel/rcutree.h > +++ b/kernel/rcutree.h > @@ -560,6 +560,7 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle, > static bool is_sysidle_rcu_state(struct rcu_state *rsp); > static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle, > unsigned long maxj); > +static void rcu_bind_gp_kthread(void); > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp); > > #endif /* #ifndef RCU_TREE_NONCORE */ > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > index ff84bed..f65d9c2 100644 > --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -2544,7 +2544,7 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle, > if (!*isidle || rdp->rsp != rcu_sysidle_state || > cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu) > return; > - /* WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); */ > + WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); but call rcu_sysidle_check_cpu() when RCU_GP_FLAG_FQS. In this time, the thread may not be bound to tick_do_timer_cpu, the WARN_ON_ONCE() may be wrong. Does any other code ensure the gp thread bound on tick_do_timer_cpu which I missed? > > /* Pick up current idle and NMI-nesting counter and check. */ > cur = atomic_read(&rdtp->dynticks_idle); > @@ -2570,6 +2570,20 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp) > } > > /* > + * Bind the grace-period kthread for the sysidle flavor of RCU to the > + * timekeeping CPU. > + */ > +static void rcu_bind_gp_kthread(void) > +{ > + int cpu = ACCESS_ONCE(tick_do_timer_cpu); > + > + if (cpu < 0 || cpu >= nr_cpu_ids) > + return; > + if (raw_smp_processor_id() != cpu) > + set_cpus_allowed_ptr(current, cpumask_of(cpu)); > +} > + > +/* > * Return a delay in jiffies based on the number of CPUs, rcu_node > * leaf fanout, and jiffies tick rate. The idea is to allow larger > * systems more time to transition to full-idle state in order to > @@ -2767,6 +2781,10 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp) > return false; > } > > +static void rcu_bind_gp_kthread(void) > +{ > +} > + > static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle, > unsigned long maxj) > {