Re: [PATCH 1.5/2] generic-smp: fix initial quiesent count.

Ingo Molnar
2009-02-14T23:32:27Z

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Sat, 2009-02-14 at 15:46 +0100, Peter Zijlstra wrote:
> > first compile, then send out...
> >
> > ---
> > Subject: generic-smp: fix initial quiesent count.
> > From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Date: Sat Feb 14 15:36:07 CET 2009
> >
> > If we start with a quiesent sequence count of 0, we'll match the initial stamp
> > values of the cfd_data, and never make any progress.
> >
> > To avoid getting stuck in this situation, start out with an increased quiesent
> > sequence count.
>
> OK, so this is not going to fix the problem in generic. Its still
> possible to end up with the original issue.
>
> We'd need a callback list to free used entries, much like regular RCU,
> this quiesent sequence count's wrapping just seems too brittle.

Ok, given that your series was hanging on a number of testboxes here,
lets go with my original simplification?

The broadcasting-to-a-lot-of-CPUs case is going to suck on any seriously
large system no matter what. It's single-IPI and few-IPI performance that
matters mostly.

Ingo