Re: vmstat: On demand vmstat workers V4

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, May 11, 2014 at 03:30:31AM +0200, Frederic Weisbecker wrote:
> On Sat, May 10, 2014 at 06:17:08PM -0700, Paul E. McKenney wrote:
> > On Sat, May 10, 2014 at 03:14:25PM +0200, Frederic Weisbecker wrote:
> > > On Sat, May 10, 2014 at 02:31:28PM +0200, Thomas Gleixner wrote:
> > > > On Sat, 10 May 2014, Frederic Weisbecker wrote:
> > > > > But I still have the plan to make the timekeeper use the full sysidle
> > > > > facility in order to adaptively get to dynticks idle.
> > > > > 
> > > > > Reminder for others: in NO_HZ_FULL, the timekeeper (always CPU 0) stays
> > > > > completely periodic. It can't enter in dynticks idle mode because it
> > > > > must maintain timekeeping on behalf of full dynticks CPUs. So that's
> > > > > a power issue.
> > > > > 
> > > > > But Paul has a feature in RCU that lets us know when all CPUs are idle
> > > > > and the timekeeper can finally sleep. Then when a full nohz CPU wakes
> > > > > up from idle, it sends an IPI to the timekeeper if needed so the latter
> > > > > restarts timekeeping maintainance.
> > > > >
> > > > > It's not complicated to add to the timer code.
> > > > > Most of the code is already there, in RCU, for a while already.
> > > > > 
> > > > > Are we keeping that direction? 
> > > > 
> > > > So the idea is that the timekeeper stays on cpu0, but if everything is
> > > > idle it is allowed to take a long nap as well. So if some other cpu
> > > > wakes up it updates timekeeping without taking over the time keeper
> > > > duty and if it has work to do, it kicks cpu0 into gear. If it just
> > > > goes back to sleep, then nothing to do.
> > 
> > Hmmm...  If RCU is supposed to ignore the fact that one of the other
> > CPUs woke up momentarily, we will need to adjust things a bit.
> 
> Maybe not that much actually.
> 
> > 
> > > Exactly! Except perhaps the last sentence "If it just goes back to sleep,
> > > then nothing to do.", I didn't think about that although this special case
> > > is quite frequent indeed when an interrupt fires on idle but no task is woken up.
> > > 
> > > Maybe I should move the code that fires the IPI to cpu0, if it is sleeping,
> > > on irq exit (the plan was to do it right away on irq enter) and fire it
> > > only if need_resched().
> > 
> > And of course if that code path contains any RCU read-side critical
> > sections, RCU absolutely cannot ignore that CPU's momentary wakeup.
> 
> Sure the core RCU still needs to know that the CPU went out of dynticks the
> time of the irq, so we keep the rcu_irq_enter/rcu_irq_exit calls.
> 
> But if the CPU only wakes up to serve an IRQ, it doesn't need to tell the RCU
> sysidle detection about it. The irq entry fixup jiffies on dynticks idle mode,
> this should be enough.

As long as you pass me in a hint so that RCU knows which case it is
dealing with.  ;-)

							Thanx, Paul

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