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

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Date:   Thu, 23 Jul 2020 14:32:36 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, Will Deacon <will@...nel.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 7/23/20 10:00 AM, Peter Zijlstra wrote:
> On Thu, Jul 09, 2020 at 12:06:13PM -0400, Waiman Long wrote:
>> We don't really need to do a pv_spinlocks_init() if pv_kick() isn't
>> supported.
> Waiman, if you cannot explain how not having kick is a sane thing, what
> are you saying here?
>
The current PPC paravirt spinlock code doesn't do any cpu kick. It does 
an equivalence of pv_wait by yielding the cpu to the lock holder only. 
The pv_spinlocks_init() is for setting up the hash table for doing 
pv_kick. If we don't need to do pv_kick, we don't need the hash table.

I am not saying that pv_kick is not needed for the PPC environment. I 
was just trying to adapt the pvqspinlock code to such an environment 
first. Further investigation on how to implement some kind of pv_kick 
will be something that we may want to do as a follow on.

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.

Cheers,
Longman