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