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

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

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.

							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
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/