Re: [PATCH 4/4 RFC] rcu: New rcu_user_enter_irq() and rcu_user_exit_irq() APIs

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Nov 28, 2011 at 01:53:23PM -0800, Josh Triplett wrote:
> 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.

Right, so I plan to use it in my nohz cpuset patchset that can be found at:

	git://github.com/fweisbec/linux-dynticks.git
		nohz/cpuset-v1

I haven't yet integrated these patches to this tree. I first need to do a big
rebase against all the changes made against dynticks API in -rcu.

I'll do that soon.