linux-kernel - Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

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Message-ID: <72f7df33-ab58-2e58-7981-cf02b6638c5b@redhat.com>
Date:   Wed, 8 Jul 2020 19:53:04 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nicholas Piggin <npiggin@...il.com>, linuxppc-dev@...ts.ozlabs.org,
        Anton Blanchard <anton@...abs.org>,
        Boqun Feng <boqun.feng@...il.com>, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

On 7/8/20 4:32 AM, Peter Zijlstra wrote:
> On Tue, Jul 07, 2020 at 11:33:45PM -0400, Waiman Long wrote:
>>  From 5d7941a498935fb225b2c7a3108cbf590114c3db Mon Sep 17 00:00:00 2001
>> From: Waiman Long <longman@...hat.com>
>> Date: Tue, 7 Jul 2020 22:29:16 -0400
>> Subject: [PATCH 2/9] locking/pvqspinlock: Introduce
>>   CONFIG_PARAVIRT_QSPINLOCKS_LITE
>>
>> Add a new PARAVIRT_QSPINLOCKS_LITE config option that allows
>> architectures to use the PV qspinlock code without the need to use or
>> implement a pv_kick() function, thus eliminating the atomic unlock
>> overhead. The non-atomic queued_spin_unlock() can be used instead.
>> The pv_wait() function will still be needed, but it can be a dummy
>> function.
>>
>> With that option set, the hybrid PV queued/unfair locking code should
>> still be able to make it performant enough in a paravirtualized
> How is this supposed to work? If there is no kick, you have no control
> over who wakes up and fairness goes out the window entirely.
>
> You don't even begin to explain...
>
I don't have a full understanding of how the PPC hypervisor work myself. 
Apparently, a cpu kick may not be needed.

This is just a test patch to see if it yields better result. It is 
subjected to further modifcation.

Cheers,
Longman