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

Ingo Molnar
2009-04-27T03:27:37Z

* 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.

Ingo