Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR

Will Deacon
2020-07-24T08:16:48Z

On Thu, Jul 23, 2020 at 08:47:59PM +0200, peterz@infradead.org wrote:
> On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long wrote:
> > BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch?
> > I will have to update the patch to fix the reported 0-day test problem, but
> > I want to collect other feedback before sending out v3.
>
> I want to say I hate it all, it adds instructions to a path we spend an
> aweful lot of time optimizing without really getting anything back for
> it.
>
> Will, how do you feel about it?

I can see it potentially being useful for debugging, but I hate the
limitation to 256 CPUs. Even arm64 is hitting that now.

Also, you're talking ~1% gains here. I think our collective time would
be better spent off reviewing the CNA series and trying to make it more
deterministic.

Will