Messages in this thread | | | Date | Mon, 28 Nov 2011 13:53:23 -0800 | From | Josh Triplett <> | Subject | Re: [PATCH 4/4 RFC] rcu: New rcu_user_enter_irq() and rcu_user_exit_irq() APIs |
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On Mon, Nov 28, 2011 at 10:24:47PM +0100, Frederic Weisbecker wrote: > A CPU running in adaptive tickless mode wants to enter into > RCU extended quiescent state while running in userspace. This > way we can shut down the tick that is usually needed on each > CPU for the needs of RCU.
Very awesome. I've wanted to see this change for a long time. Thanks!
> Typically, RCU enters the extended quiescent state when we resume > to userspace through a syscall or exception exit, this is done > using rcu_user_enter(). Then RCU exit this state by calling > rcu_user_exit() from syscall or exception entry. > > However there are two other points where we may want to enter > or exit this state. Some remote CPU may require a tickless CPU > to restart its tick for any reason and send it an IPI for > this purpose. As we restart the tick, we don't want to resume > from the IPI in RCU extended quiescent state anymore. > Similarly we may stop the tick from an interrupt in userspace and > we need to be able to enter RCU extended quiescent state when we > resume from this interrupt to userspace. > > To these ends, we provide two new APIs: > > - rcu_user_enter_irq(). This must be called from a non-nesting > interrupt betwenn rcu_irq_enter() and rcu_irq_exit(). > After the irq calls rcu_irq_exit(), we'll run into RCU extended > quiescent state. > > - rcu_user_exit_irq(). This must be called from a non-nesting > interrupt, interrupting an RCU extended quiescent state, and > between rcu_irq_enter() and rcu_irq_exit(). After the irq calls > rcu_irq_exit(), we'll prevent from resuming the RCU extended > quiescent.
It would help to see the corresponding patches making use of this new API.
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Josh Triplett <josh@joshtriplett.org> > --- > kernel/rcutree.c | 24 ++++++++++++++++++++++++ > 1 files changed, 24 insertions(+), 0 deletions(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index 00a9fba..a7906c9 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -402,6 +402,18 @@ void rcu_user_enter(void) > __rcu_idle_enter(); > } > > +void rcu_user_enter_irq(void) > +{ > + unsigned long flags; > + struct rcu_dynticks *rdtp; > + > + local_irq_save(flags); > + rdtp = &__get_cpu_var(rcu_dynticks); > + WARN_ON_ONCE(rdtp->dynticks_nesting == 1); > + rdtp->dynticks_nesting = 1; > + local_irq_restore(flags); > +} > + > /** > * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle > * > @@ -503,6 +515,18 @@ void rcu_user_exit(void) > __rcu_idle_exit(); > } > > +void rcu_user_exit_irq(void) > +{ > + unsigned long flags; > + struct rcu_dynticks *rdtp; > + > + local_irq_save(flags); > + rdtp = &__get_cpu_var(rcu_dynticks); > + WARN_ON_ONCE(rdtp->dynticks_nesting != 1); > + rdtp->dynticks_nesting = (LLONG_MAX / 2) + 1; > + local_irq_restore(flags); > +} > +
Any chance that either of these two needs a memory barrier of some kind, to prevent leakage of operations from between them? Or can you count on no RCU-protected operations occurring during (or leaking into) the extended quiescent state?
- Josh Triplett
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