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

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Date:   Fri, 24 Jul 2020 09:16:48 +0100
From:   Will Deacon <will@...nel.org>
To:     peterz@...radead.org
Cc:     Waiman Long <longman@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Anton Blanchard <anton@...abs.org>,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks
 for SPLPAR

On Thu, Jul 23, 2020 at 08:47:59PM +0200, peterz@...radead.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