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Date: Wed, 23 May 2012 16:33:47 +0200
From: Frederic Weisbecker <fweisbec@...il.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
linaro-sched-sig@...ts.linaro.org,
Alessio Igor Bogani <abogani@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Avi Kivity <avi@...hat.com>,
Chris Metcalf <cmetcalf@...era.com>,
Christoph Lameter <cl@...ux.com>,
Daniel Lezcano <daniel.lezcano@...aro.org>,
Geoff Levand <geoff@...radead.org>,
Gilad Ben Yossef <gilad@...yossef.com>,
Hakan Akkan <hakanakkan@...il.com>,
Ingo Molnar <mingo@...nel.org>, Kevin Hilman <khilman@...com>,
Max Krasnyansky <maxk@...lcomm.com>,
Peter Zijlstra <peterz@...radead.org>,
Stephen Hemminger <shemminger@...tta.com>,
Steven Rostedt <rostedt@...dmis.org>,
Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 39/41] rcu: Switch to extended quiescent state in
userspace from nohz cpuset
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 <fweisbec@...il.com>
> > Cc: Alessio Igor Bogani <abogani@...nel.org>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Avi Kivity <avi@...hat.com>
> > Cc: Chris Metcalf <cmetcalf@...era.com>
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> > Cc: Geoff Levand <geoff@...radead.org>
> > Cc: Gilad Ben Yossef <gilad@...yossef.com>
> > Cc: Hakan Akkan <hakanakkan@...il.com>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Kevin Hilman <khilman@...com>
> > Cc: Max Krasnyansky <maxk@...lcomm.com>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Stephen Hemminger <shemminger@...tta.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Sven-Thorsten Dietrich <thebigcorporation@...il.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > ---
> > 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
> >
>
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