Messages in this thread | | | Date | Tue, 22 May 2012 16:04:56 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 39/41] rcu: Switch to extended quiescent state in userspace from nohz cpuset |
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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.
And I found out one reason: WARN_ON_ONCE(rcu_is_cpu_idle()) works only if CONFIG_PROVE_RCU. I am fixing this.
Thanx, Paul
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > > Cc: Alessio Igor Bogani <abogani@kernel.org> > > Cc: Andrew Morton <akpm@linux-foundation.org> > > Cc: Avi Kivity <avi@redhat.com> > > Cc: Chris Metcalf <cmetcalf@tilera.com> > > Cc: Christoph Lameter <cl@linux.com> > > Cc: Daniel Lezcano <daniel.lezcano@linaro.org> > > Cc: Geoff Levand <geoff@infradead.org> > > Cc: Gilad Ben Yossef <gilad@benyossef.com> > > Cc: Hakan Akkan <hakanakkan@gmail.com> > > Cc: Ingo Molnar <mingo@kernel.org> > > Cc: Kevin Hilman <khilman@ti.com> > > Cc: Max Krasnyansky <maxk@qualcomm.com> > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Stephen Hemminger <shemminger@vyatta.com> > > Cc: Steven Rostedt <rostedt@goodmis.org> > > Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com> > > Cc: Thomas Gleixner <tglx@linutronix.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? > > > } > > } > > > > -- > > 1.7.5.4 > >
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