Re: [PATCH RFC] v2 expedited "big hammer" RCU grace periods

Paul E. McKenney
2009-04-27T13:22:09Z

On Mon, Apr 27, 2009 at 05:26:39AM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>
> > On Sun, Apr 26, 2009 at 10:22:55PM +0200, Ingo Molnar wrote:
> > >
> > > * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> > >
> > > > * Ingo Molnar (mingo@elte.hu) wrote:
> > > > >
> > > > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > > > >
> > > > > > Second cut of "big hammer" expedited RCU grace periods, but only
> > > > > > for rcu_bh. This creates another softirq vector, so that entering
> > > > > > this softirq vector will have forced an rcu_bh quiescent state (as
> > > > > > noted by Dave Miller). Use smp_call_function() to invoke
> > > > > > raise_softirq() on all CPUs in order to cause this to happen.
> > > > > > Track the CPUs that have passed through a quiescent state (or gone
> > > > > > offline) with a cpumask.
> > > > >
> > > > > hm, i'm still asking whether doing this would be simpler via a
> > > > > reschedule vector - which not only is an existing facility but also
> > > > > forces all RCU domains through a quiescent state - not just bh-RCU
> > > > > participants.
> > > > >
> > > > > Triggering a new softirq is in no way simpler that doing an SMP
> > > > > cross-call - in fact softirqs are a finite resource so using some
> > > > > other facility would be preferred.
> > > > >
> > > > > Am i missing something?
> > > > >
> > > >
> > > > I think the reason for this whole thread is that waiting for rcu
> > > > quiescent state, when called many times e.g. in multiple iptables
> > > > invokations, takes too longs (5 seconds to load the netfilter
> > > > rules at boot). [...]
> > >
> > > I'm aware of the problem space.
> > >
> > > I was suggesting that to trigger the quiescent state and to wait for
> > > it to propagate it would be enough to reuse the reschedule
> > > mechanism.
> > >
> > > It would be relatively straightforward: first a send-reschedule then
> > > do a wait_task_context_switch() on rq->curr - both are existing
> > > primitives. (a task reference has to be taken but that's pretty much
> > > all)
> >
> > Well, one reason I didn't take this approach was that I didn't
> > happen to think of it. ;-)
> >
> > Also that I hadn't heard of wait_task_context_switch().
> >
> > Hmmm... Looking for wait_task_context_switch(). OK, found it.
> >
> > It looks to me that this primitive won't return until the
> > scheduler actually decides to run something else. We instead need
> > to have something that stops waiting once the CPU enters the
> > scheduler, hence the previous thought of making rcu_qsctr_inc() do
> > a bit of extra work.
> >
> > This would be a way of making an expedited RCU-sched across all
> > RCU implementations. As noted in the earlier email, it would not
> > handle RCU or RCU-bh in a -rt kernel.
> >
> > > By the time wait_task_context_switch() returns from the last CPU
> > > we know that the quiescent state has passed.
> >
> > We would want to wait for all of the CPUs in parallel, though,
> > wouldn't we? Seems that we would not want to wait for the last
> > CPU to do another trip through the scheduler if it had already
> > passed through the scheduler while we were waiting on the earlier
> > CPUs.
> >
> > So it seems like we would still want a two-pass approach -- one
> > pass to capture the current state, the second pass to wait for the
> > state to change.
>
> I think waiting in parallel is still possible (first kick all tasks,
> then make sure all tasks have left the CPU at least once).
>
> The busy-waiting in wait_task_context_switch() is indeed a problem -
> but perhaps that could be refactored to be a migration-thread driven
> wait_for_completion() + complete() cycle? It could be driven by
> preempt notifiers perhaps - and become zero-cost.

Hmmm... It would need to be informed of the quiescent state even if
that quiescent state did not result in a preemption.

But you are right -- I do need to expedite RCU, not just RCU-bh,
especially given that the boot-speed guys are starting to see grace
periods as a measureable fraction of the boot time. I will take another
pass at this.

Thanx, Paul