Messages in this thread | | | Date | Wed, 7 Sep 2011 07:22:33 -0700 | From | "Paul E. McKenney" <> | 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.
But I get RCU stall warnings when running it on powerpc on top of the patch set at:
https://lkml.org/lkml/2011/9/7/64
At first glance, it appears that CPUs are entering dyntick-idle mode without RCU being informed. Any thoughts on diagnostics?
Thanx, Paul
> > 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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