Messages in this thread | | | Date | Mon, 21 Nov 2011 02:46:58 +0100 | Subject | Re: [PATCH] nohz: Remove tick_nohz_idle_enter_norcu() / tick_nohz_idle_exit_norcu() | From | Frederic Weisbecker <> |
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2011/11/19 Paul E. McKenney <paulmck@linux.vnet.ibm.com>: > On Thu, Nov 17, 2011 at 05:03:44PM -0800, Paul E. McKenney wrote: >> On Thu, Nov 17, 2011 at 12:11:34PM -0800, Josh Triplett wrote: >> > On Thu, Nov 17, 2011 at 06:48:14PM +0100, Frederic Weisbecker wrote: >> > > Those two APIs were provided to optimize the calls of >> > > tick_nohz_idle_enter() and rcu_idle_enter() into a single >> > > irq disabled section. This way no interrupt happening in-between would >> > > needlessly process any RCU job. >> > > >> > > Now we are talking about an optimization for which benefits >> > > have yet to be measured. Let's start simple and completely decouple >> > > idle rcu and dyntick idle logics to simplify. >> > > >> > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> >> > > Cc: Ingo Molnar <mingo@redhat.com> >> > > Cc: Thomas Gleixner <tglx@linutronix.de> >> > > Cc: Peter Zijlstra <peterz@infradead.org> >> > > Cc: Josh Triplett <josh@joshtriplett.org> >> > >> > Reviewed-by: Josh Triplett <josh@joshtriplett.org> >> >> Merged, thank you both! > > And here is a patch on top of yours to allow nesting of rcu_idle_enter() > and rcu_idle_exit(). Thoughts? > > Thanx, Paul > > ------------------------------------------------------------------------ > > rcu: Allow nesting of rcu_idle_enter() and rcu_idle_exit() > > Running user tasks in dyntick-idle mode requires RCU to undergo > an idle-to-non-idle transition on each entry into the kernel, and > vice versa on each exit from the kernel. However, situations where > user tasks cannot run in dyntick-idle mode (for example, when there > is more than one runnable task on the CPU in question) also require > RCU to undergo an idle-to-non-idle transition when coming out of the > idle loop (and vice versa when entering the idle loop).
Not sure what you mean about the idle loop with the dyntick-idle mode we can't enter when we resume to userspace with more than one task in the runqueue.
> In this case, > RCU would see one idle-to-non-idle transition when the task became > runnable, and another when the task executed a system call.
I'm a bit confused with this changelog.
What can happen with the adaptive tickless thing is:
- When we resume to userspace after a syscall/irq/exception and we are not in RCU extended quiescent state, then switch to it. We may call it RCU idle mode I guess but that may start to be confusing. So this may involve several kind of nesting. From a single rcu_idle_enter() to more complicated scenario if we switch to RCU extended qs from an an interrupt: rcu_idle_exit() is called on entry of the irq, rcu_idle_enter() is called in the middle then finally a last call to rcu_idle_enter() in the irq exit at which point only we want the RCU extended qs to be effective.
- We may also exit that RCU extended qs state by involving other funny nesting. We have the simple syscall enter that just calls rcu_idle_exit() if we were in userspace in RCU extended qs. We may also receive an IPI that enqueues a new task, in which case we may exit the RCU extended quiescent from the irq with the following nesting: rcu_idle_exit() on irq entry, then another call to rcu_idle_exit() to prevent from resuming the RCU extended quiescent state when we come back to userspace and finally the rcu_idle_enter() in the irq exit.
Is that what you had in mind?
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