Messages in this thread | | | Date | Tue, 6 Sep 2011 16:58:03 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 1/4] nohz: Split extended quiescent state handling from nohz switch |
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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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