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    SubjectRe: [PATCH 1/4] nohz: Split extended quiescent state handling from nohz switch
    On Sun, Sep 04, 2011 at 04:36:43PM -0700, Paul E. McKenney wrote:
    > 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.

    Right. But I wanted this patch to only perform the API conversion, then
    later pushdown the rcu_enter_nohz() things.

    > > 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.

    Right

    <snip>
    > > +/**
    > > + * 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.

    Better yeah, I'll use your version ;)

    > > + */
    > > +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.

    Right, but to keep things clear, as long as we call tick_nohz_enter_idle(false)
    and rcu_enter_nohz() later, we should keep the same mirroring with rcu_exit_nohz()
    and tick_nohz_exit_idle(false).


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