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    SubjectRe: [PATCH 1/4] nohz: Split extended quiescent state handling from nohz switch
    On Sat, Aug 20, 2011 at 07:30:49PM +0200, Frederic Weisbecker wrote:
    > It is assumed that rcu won't be used once we switch to tickless
    > mode and until we restart the tick. However this is not always
    > true, as in x86-64 where we dereference the idle notifiers after
    > the tick is stopped.
    >
    > To prepare for fixing this, split the tickless mode switching and
    > RCU extended quiescent state logics.
    > Make tick_nohz_stop/restart_sched_tick() RCU agnostic but provide
    > a new pair of APIs tick_nohz_enter/exit_idle() that keep the
    > old behaviour by handling both the nohz mode and RCU extended
    > quiescent states, then convert every archs to use these.
    >
    > Archs that want to switch to RCU extended QS to some custom points
    > can do it later by changing the parameter in tick_nohz_enter,exit_idle()
    > to false and call rcu_enter,exit() separately.

    This approach looks quite good to me! A few comments below.

    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: David Miller <davem@davemloft.net>
    > Cc: Chris Metcalf <cmetcalf@tilera.com>
    > Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
    > Cc: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
    > Cc: Mike Frysinger <vapier@gentoo.org>
    > Cc: Ralf Baechle <ralf@linux-mips.org>
    > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Cc: Ingo Molnar <mingo@elte.hu>
    > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: H. Peter Anvin <hpa@zytor.com>
    > Cc: Russell King <linux@arm.linux.org.uk>
    > Cc: Paul Mackerras <paulus@samba.org>
    > Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
    > Cc: Paul Mundt <lethal@linux-sh.org>
    > ---

    [ . . . ]

    > diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
    > index 39a2baa..b30ddf1 100644
    > --- a/arch/powerpc/kernel/idle.c
    > +++ b/arch/powerpc/kernel/idle.c
    > @@ -56,7 +56,7 @@ void cpu_idle(void)
    >
    > set_thread_flag(TIF_POLLING_NRFLAG);
    > while (1) {
    > - tick_nohz_stop_sched_tick(1);
    > + tick_nohz_enter_idle(true);

    This needs to pass false, as some variants of powerpc use event tracing
    (which in turn uses RCU) just before returning to the hypervisor.

    > while (!need_resched() && !cpu_should_die()) {
    > ppc64_runlatch_off();
    >
    > @@ -93,7 +93,7 @@ void cpu_idle(void)
    >
    > HMT_medium();
    > ppc64_runlatch_on();
    > - tick_nohz_restart_sched_tick();
    > + tick_nohz_exit_idle(true);

    As does this, as some variants of powerpc use event tracing just after
    the hypervisor returns control to the OS.

    > preempt_enable_no_resched();
    > if (cpu_should_die())
    > cpu_die();
    > diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c
    > index c25a081..aa2be7d 100644
    > --- a/arch/powerpc/platforms/iseries/setup.c
    > +++ b/arch/powerpc/platforms/iseries/setup.c
    > @@ -562,7 +562,7 @@ static void yield_shared_processor(void)
    > static void iseries_shared_idle(void)
    > {
    > while (1) {
    > - tick_nohz_stop_sched_tick(1);
    > + tick_nohz_enter_idle(true);

    But I don't know enough about iseries to know whether or not this works.

    > while (!need_resched() && !hvlpevent_is_pending()) {
    > local_irq_disable();
    > ppc64_runlatch_off();
    > @@ -576,7 +576,7 @@ static void iseries_shared_idle(void)
    > }
    >
    > ppc64_runlatch_on();
    > - tick_nohz_restart_sched_tick();
    > + tick_nohz_exit_idle(true);
    >
    > if (hvlpevent_is_pending())
    > process_iSeries_events();
    > @@ -592,7 +592,7 @@ static void iseries_dedicated_idle(void)
    > set_thread_flag(TIF_POLLING_NRFLAG);
    >
    > while (1) {
    > - tick_nohz_stop_sched_tick(1);
    > + tick_nohz_enter_idle(true);
    > if (!need_resched()) {
    > while (!need_resched()) {
    > ppc64_runlatch_off();
    > @@ -609,7 +609,7 @@ static void iseries_dedicated_idle(void)
    > }
    >
    > ppc64_runlatch_on();
    > - tick_nohz_restart_sched_tick();
    > + tick_nohz_exit_idle(true);
    > preempt_enable_no_resched();
    > schedule();
    > preempt_disable();

    [ . . . ]

    > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    > index eb98e55..609bb20 100644
    > --- a/kernel/time/tick-sched.c
    > +++ b/kernel/time/tick-sched.c
    > @@ -253,12 +253,13 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
    > * Called either from the idle loop or from irq_exit() when an idle period was
    > * just interrupted by an interrupt which did not cause a reschedule.
    > */
    > -void tick_nohz_stop_sched_tick(int inidle)
    > +bool tick_nohz_stop_sched_tick(int inidle)
    > {
    > unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
    > struct tick_sched *ts;
    > ktime_t last_update, expires, now;
    > struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
    > + int stopped = false;
    > u64 time_delta;
    > int cpu;
    >
    > @@ -405,7 +406,7 @@ void tick_nohz_stop_sched_tick(int inidle)
    > ts->idle_tick = hrtimer_get_expires(&ts->sched_timer);
    > ts->tick_stopped = 1;
    > ts->idle_jiffies = last_jiffies;
    > - rcu_enter_nohz();
    > + stopped = true;
    > }
    >
    > ts->idle_sleeps++;
    > @@ -445,6 +446,24 @@ out:
    > ts->sleep_length = ktime_sub(dev->next_event, now);
    > end:
    > local_irq_restore(flags);
    > +
    > + return stopped;
    > +}
    > +
    > +
    > +/**
    > + * tick_nohz_enter_idle - stop the tick from the idle task
    > + * @rcu_ext_qs: enter rcu dynticks idle mode
    > + *
    > + * When an arch doesn't make any use of rcu read side critical section
    > + * between tick_nohz_enter_idle() and tick_nohz_exit_idle() it can set
    > + * rcu_ext_qs to 1. Otherwise it needs to call rcu_enter_nohz() itself
    > + * later before the CPU goes to sleep.

    How about something like this?

    + * When an arch doesn't make any use of rcu read side critical section
    + * between tick_nohz_enter_idle() and the time the CPU is put to sleep,
    + * it can set rcu_ext_qs to true. Otherwise it needs set rcu_ext_qs to
    + * false, and then to call rcu_enter_nohz() itself later, but before the
    + * CPU goes to sleep and after its last use of RCU.

    > + */
    > +void tick_nohz_enter_idle(bool rcu_ext_qs)
    > +{
    > + if (tick_nohz_stop_sched_tick(1) && rcu_ext_qs)
    > + rcu_enter_nohz();
    > }
    >
    > /**
    > @@ -486,11 +505,12 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
    > }
    >
    > /**
    > - * tick_nohz_restart_sched_tick - restart the idle tick from the idle task
    > + * tick_nohz_exit_idle - restart the idle tick from the idle task
    > + * @rcu_ext_qs: exit rcu dynticks idle mode
    > *
    > * Restart the idle tick when the CPU is woken up from idle

    How about something like the following?

    + * When an arch doesn't make any use of rcu read side critical section
    + * between the time the CPU wakes up and tick_nohz_exit_idle(), it can set
    + * rcu_ext_qs to true. Otherwise it needs set rcu_ext_qs to false, and it
    + * must also have called rcu_enter_nohz() itself earlier, after the CPU
    + * was awakened, but before its first use of RCU.

    > */
    > -void tick_nohz_restart_sched_tick(void)
    > +void tick_nohz_exit_idle(bool rcu_ext_qs)
    > {
    > int cpu = smp_processor_id();
    > struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
    > @@ -514,7 +534,8 @@ void tick_nohz_restart_sched_tick(void)
    >
    > ts->inidle = 0;
    >
    > - rcu_exit_nohz();
    > + if (rcu_ext_qs)
    > + rcu_exit_nohz();
    >
    > /* Update jiffies first */
    > select_nohz_load_balancer(0);
    > --
    > 1.7.5.4
    >


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