From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933482Ab2EWOd5 (ORCPT ); Wed, 23 May 2012 10:33:57 -0400 Received: from mail-qc0-f174.google.com ([209.85.216.174]:63006 "EHLO mail-qc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754086Ab2EWOdz (ORCPT ); Wed, 23 May 2012 10:33:55 -0400 Date: Wed, 23 May 2012 16:33:47 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: LKML , linaro-sched-sig@lists.linaro.org, Alessio Igor Bogani , Andrew Morton , Avi Kivity , Chris Metcalf , Christoph Lameter , Daniel Lezcano , Geoff Levand , Gilad Ben Yossef , Hakan Akkan , Ingo Molnar , Kevin Hilman , Max Krasnyansky , Peter Zijlstra , Stephen Hemminger , Steven Rostedt , Sven-Thorsten Dietrich , Thomas Gleixner Subject: Re: [PATCH 39/41] rcu: Switch to extended quiescent state in userspace from nohz cpuset Message-ID: <20120523143344.GH1663@somewhere> References: <1335830115-14335-1-git-send-email-fweisbec@gmail.com> <1335830115-14335-40-git-send-email-fweisbec@gmail.com> <20120522183630.GF8087@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120522183630.GF8087@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 22, 2012 at 11:36:30AM -0700, Paul E. McKenney wrote: > On Tue, May 01, 2012 at 01:55:13AM +0200, Frederic Weisbecker wrote: > > When we switch to adaptive nohz mode and we run in userspace, > > we can still receive IPIs from the RCU core if a grace period > > has been started by another CPU because we need to take part > > of its completion. > > > > However running in userspace is similar to that of running in > > idle because we don't make use of RCU there, thus we can be > > considered as running in RCU extended quiescent state. The > > benefit when running into that mode is that we are not > > anymore disturbed by needless IPIs coming from the RCU core. > > > > To perform this, we just to use the RCU extended quiescent state > > APIs on the following points: > > > > - kernel exit or tick stop in userspace: here we switch to extended > > quiescent state because we run in userspace without the tick. > > > > - kernel entry or tick restart: here we exit the extended quiescent > > state because either we enter the kernel and we may make use of RCU > > read side critical section anytime, or we need the timer tick for some > > reason and that takes care of RCU grace period in a traditional way. > > One FIXME question below. > > Thanx, Paul > > > Signed-off-by: Frederic Weisbecker > > Cc: Alessio Igor Bogani > > Cc: Andrew Morton > > Cc: Avi Kivity > > Cc: Chris Metcalf > > Cc: Christoph Lameter > > Cc: Daniel Lezcano > > Cc: Geoff Levand > > Cc: Gilad Ben Yossef > > Cc: Hakan Akkan > > Cc: Ingo Molnar > > Cc: Kevin Hilman > > Cc: Max Krasnyansky > > Cc: Paul E. McKenney > > Cc: Peter Zijlstra > > Cc: Stephen Hemminger > > Cc: Steven Rostedt > > Cc: Sven-Thorsten Dietrich > > Cc: Thomas Gleixner > > --- > > include/linux/tick.h | 3 +++ > > kernel/time/tick-sched.c | 27 +++++++++++++++++++++++++-- > > 2 files changed, 28 insertions(+), 2 deletions(-) > > > > diff --git a/include/linux/tick.h b/include/linux/tick.h > > index 3c31d6e..e2a49ad 100644 > > --- a/include/linux/tick.h > > +++ b/include/linux/tick.h > > @@ -153,6 +153,8 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; } > > # endif /* !NO_HZ */ > > > > #ifdef CONFIG_CPUSETS_NO_HZ > > +DECLARE_PER_CPU(int, nohz_task_ext_qs); > > + > > extern void tick_nohz_enter_kernel(void); > > extern void tick_nohz_exit_kernel(void); > > extern void tick_nohz_enter_exception(struct pt_regs *regs); > > @@ -160,6 +162,7 @@ extern void tick_nohz_exit_exception(struct pt_regs *regs); > > extern void tick_nohz_check_adaptive(void); > > extern void tick_nohz_pre_schedule(void); > > extern void tick_nohz_post_schedule(void); > > +extern void tick_nohz_cpu_exit_qs(void); > > extern bool tick_nohz_account_tick(void); > > extern void tick_nohz_flush_current_times(bool restart_tick); > > #else /* !CPUSETS_NO_HZ */ > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > index 8217409..b15ab5e 100644 > > --- a/kernel/time/tick-sched.c > > +++ b/kernel/time/tick-sched.c > > @@ -565,10 +565,13 @@ static void tick_nohz_cpuset_stop_tick(struct tick_sched *ts) > > > > if (!was_stopped && ts->tick_stopped) { > > WARN_ON_ONCE(ts->saved_jiffies_whence != JIFFIES_SAVED_NONE); > > - if (user) > > + if (user) { > > ts->saved_jiffies_whence = JIFFIES_SAVED_USER; > > - else if (!current->mm) > > + __get_cpu_var(nohz_task_ext_qs) = 1; > > + rcu_user_enter_irq(); > > + } else if (!current->mm) { > > ts->saved_jiffies_whence = JIFFIES_SAVED_SYS; > > + } > > > > ts->saved_jiffies = jiffies; > > set_thread_flag(TIF_NOHZ); > > @@ -899,6 +902,8 @@ void tick_check_idle(int cpu) > > } > > > > #ifdef CONFIG_CPUSETS_NO_HZ > > +DEFINE_PER_CPU(int, nohz_task_ext_qs); > > + > > void tick_nohz_exit_kernel(void) > > { > > unsigned long flags; > > @@ -922,6 +927,9 @@ void tick_nohz_exit_kernel(void) > > ts->saved_jiffies = jiffies; > > ts->saved_jiffies_whence = JIFFIES_SAVED_USER; > > > > + __get_cpu_var(nohz_task_ext_qs) = 1; > > + rcu_user_enter(); > > + > > local_irq_restore(flags); > > } > > > > @@ -940,6 +948,11 @@ void tick_nohz_enter_kernel(void) > > return; > > } > > > > + if (__get_cpu_var(nohz_task_ext_qs) == 1) { > > + __get_cpu_var(nohz_task_ext_qs) = 0; > > + rcu_user_exit(); > > + } > > + > > WARN_ON_ONCE(ts->saved_jiffies_whence != JIFFIES_SAVED_USER); > > > > delta_jiffies = jiffies - ts->saved_jiffies; > > @@ -951,6 +964,14 @@ void tick_nohz_enter_kernel(void) > > local_irq_restore(flags); > > } > > > > +void tick_nohz_cpu_exit_qs(void) > > +{ > > + if (__get_cpu_var(nohz_task_ext_qs)) { > > + rcu_user_exit_irq(); > > + __get_cpu_var(nohz_task_ext_qs) = 0; > > + } > > +} > > + > > void tick_nohz_enter_exception(struct pt_regs *regs) > > { > > if (user_mode(regs)) > > @@ -986,6 +1007,7 @@ static void tick_nohz_restart_adaptive(void) > > tick_nohz_flush_current_times(true); > > tick_nohz_restart_sched_tick(); > > clear_thread_flag(TIF_NOHZ); > > + tick_nohz_cpu_exit_qs(); > > } > > > > void tick_nohz_check_adaptive(void) > > @@ -1023,6 +1045,7 @@ void tick_nohz_pre_schedule(void) > > if (ts->tick_stopped) { > > tick_nohz_flush_current_times(true); > > clear_thread_flag(TIF_NOHZ); > > + /* FIXME: warn if we are in RCU idle mode */ > > This would be WARN_ON_ONCE(rcu_is_cpu_idle()) or some such, correct? Yeah indeed. I'll add that. > > > } > > } > > > > -- > > 1.7.5.4 > > >