Re: [PATCH 31/32] rcu: Switch to extended quiescent state in userspace from nohz cpuset

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Aug 16, 2011 at 01:44:15PM -0700, Paul E. McKenney wrote:
> On Mon, Aug 15, 2011 at 05:52:28PM +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.
> > 
> > TODO: hook into do_notify_resume() because we may have called
> > rcu_enter_nohz() from syscall exit hook, but we might call
> > do_notify_resume() right after, which may use RCU.
> 
> I don't see exactly how the exception path works, but this does reassure
> me a bit on the syscall path.

On the syscall path we directly call tick_nohz_enter,exit_kernel() and that
takes care of all the rcu trickies.

The exception paths call tick_nohz_enter,exit_exception() which are
essentially conditional wrappers around tick_nohz_enter,exit_kernel()
after checking user_mode(regs). Hmm now I realize I can't rely
on user_mode() to know if we should exit or not rcu extended quiescent state
because even if user_mode(regs) != 1, we may not have yet called
the syscall exit hook and thus not exited rcu quiescent state. So
tick_nohz_enter_exception() may forget to call rcu_exit_nohz() sometimes.

I need to fix that.

> 
> 							Thanx, Paul
> 
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Anton Blanchard <anton@au1.ibm.com>
> > Cc: Avi Kivity <avi@redhat.com>
> > Cc: Ingo Molnar <mingo@elte.hu>
> > Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> > Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Paul Menage <menage@google.com>
> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Stephen Hemminger <shemminger@vyatta.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Tim Pepper <lnxninja@linux.vnet.ibm.com>
> > ---
> >  include/linux/tick.h     |    2 ++
> >  kernel/sched.c           |    1 +
> >  kernel/time/tick-sched.c |   21 +++++++++++++++++++++
> >  3 files changed, 24 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index 9d0270e..4e7555f 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -138,12 +138,14 @@ extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
> > 
> >  #ifdef CONFIG_CPUSETS_NO_HZ
> >  DECLARE_PER_CPU(int, task_nohz_mode);
> > +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);
> >  extern void tick_nohz_exit_exception(struct pt_regs *regs);
> >  extern int tick_nohz_adaptive_mode(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/sched.c b/kernel/sched.c
> > index 2bcd456..576d0bf 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -2504,6 +2504,7 @@ static void cpuset_nohz_restart_tick(void)
> >  	__get_cpu_var(task_nohz_mode) = 0;
> >  	tick_nohz_restart_sched_tick();
> >  	clear_thread_flag(TIF_NOHZ);
> > +	tick_nohz_cpu_exit_qs();
> >  }
> > 
> >  void cpuset_update_nohz(void)
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 9a2ba5b..b611b77 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -757,6 +757,7 @@ 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)
> >  {
> > @@ -783,6 +784,9 @@ 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_enter_nohz();
> 
> OK, I was wondering how this was going to work if RCU didn't
> know about kernel entry/exit.  Whew!!!  ;-)
> 
> > +
> >  	local_irq_restore(flags);
> >  }
> > 
> > @@ -799,6 +803,11 @@ 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_exit_nohz();
> > +	}
> > +
> >  	ts = &__get_cpu_var(tick_cpu_sched);
> > 
> >  	WARN_ON_ONCE(ts->saved_jiffies_whence == JIFFIES_SAVED_SYS);
> > @@ -814,6 +823,16 @@ void tick_nohz_enter_kernel(void)
> >  	local_irq_restore(flags);
> >  }
> > 
> > +void tick_nohz_cpu_exit_qs(void)
> > +{
> > +	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> > +
> > +	if (__get_cpu_var(nohz_task_ext_qs)) {
> > +		rcu_exit_nohz();
> > +		__get_cpu_var(nohz_task_ext_qs) = 0;
> > +	}
> > +}
> > +
> >  void tick_nohz_enter_exception(struct pt_regs *regs)
> >  {
> >  	if (user_mode(regs))
> > @@ -858,6 +877,8 @@ static void tick_nohz_cpuset_stop_tick(int user)
> >  		if (user) {
> >  			ts->saved_jiffies_whence = JIFFIES_SAVED_USER;
> >  			ts->saved_jiffies = jiffies;
> > +			__get_cpu_var(nohz_task_ext_qs) = 1;
> > +			rcu_enter_nohz();
> 
> When entering an exception, shouldn't we call rcu_exit_nohz() rather
> than rcu_exit_nohz()?  Or is this a "didn't really mean an exception"
> code path?
> 
> >  		} else if (!current->mm) {
> >  			ts->saved_jiffies_whence = JIFFIES_SAVED_SYS;
> >  			ts->saved_jiffies = jiffies;
> > -- 
> > 1.7.5.4
> > 
> > --
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