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

Ingo Molnar
2009-04-26T20:23:37Z

* 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)

By the time wait_task_context_switch() returns from the last CPU we
know that the quiescent state has passed.

Ingo