Re: [PATCH 39/41] rcu: Switch to extended quiescent state in userspace from nohz cpuset

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@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
>