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Date: Tue, 22 May 2012 11:36:30 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.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 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?
> }
> }
>
> --
> 1.7.5.4
>
--
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