Re: [tip:perf/core] perf: Per-pmu-per-cpu contexts

Paul E. McKenney
2010-09-10T16:05:35Z

On Fri, Sep 10, 2010 at 05:46:34PM +0200, Peter Zijlstra wrote:
> On Fri, 2010-09-10 at 08:37 -0700, Paul E. McKenney wrote:
> > > So, you say below that it works because synchronize_srcu(), that
> > > waits for qs after touching pmus, implies synchronize_sched(), right?
> >
> > Ook... My current plans to fold SRCU into TREE_RCU would invalidate
> > this assumption.
> >
> > Maybe we need some sort of primitive that concurrently waits for
> > multiple types of RCU grace periods?
>
> Nah, but I was thinking that any kind of preemptible rcu sync would
> imply a sched rcu sync.

Ah!

Although disabling interrupts will block preemptible RCU grace periods
in current implementations (but please don't rely on this!), disabling
preemption will -not- block preemptible RCU grace periods, even given
current TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations. Current
SRCU grace periods are blocked by disabling preemption, but folding it
into the tree/tiny implementations would make SRCU grace periods be no
longer blocked by disabling preemption.

This might change if RCU priority boosting is enabled, due to RCU
grace-period computation and callback invocation moving to a kthread,
but I won't have the guts to make TREE_RCU use kthread by default for
some time. (Probably a year or so trouble-free experience with RCU
priority boosting/kthreads.)

> If not strictly implied I'd have no problem simply writing:
>
> synchronize_rcu_sched();
> synchronize_srcu();

If that works for you, then we are set! The only reason to introduce
a combined primitive would be if the latency of the above was too large.

Thanx, Paul