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

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Date:   Tue, 7 Jul 2020 23:33:45 -0400
From:   Waiman Long <longman@...hat.com>
To:     Nicholas Piggin <npiggin@...il.com>, linuxppc-dev@...ts.ozlabs.org
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        virtualization@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

On 7/7/20 1:57 AM, Nicholas Piggin wrote:
> Yes, powerpc could certainly get more performance out of the slow
> paths, and then there are a few parameters to tune.
>
> We don't have a good alternate patching for function calls yet, but
> that would be something to do for native vs pv.
>
> And then there seem to be one or two tunable parameters we could
> experiment with.
>
> The paravirt locks may need a bit more tuning. Some simple testing
> under KVM shows we might be a bit slower in some cases. Whether this
> is fairness or something else I'm not sure. The current simple pv
> spinlock code can do a directed yield to the lock holder CPU, whereas
> the pv qspl here just does a general yield. I think we might actually
> be able to change that to also support directed yield. Though I'm
> not sure if this is actually the cause of the slowdown yet.

Regarding the paravirt lock, I have taken a further look into the 
current PPC spinlock code. There is an equivalent of pv_wait() but no 
pv_kick(). Maybe PPC doesn't really need that. Attached are two 
additional qspinlock patches that adds a CONFIG_PARAVIRT_QSPINLOCKS_LITE 
option to not require pv_kick(). There is also a fixup patch to be 
applied after your patchset.

I don't have access to a PPC LPAR with shared processor at the moment, 
so I can't test the performance of the paravirt code. Would you mind 
adding my patches and do some performance test on your end to see if it 
gives better result?

Thanks,
Longman


View attachment "0001-locking-pvqspinlock-Code-relocation-and-extraction.patch" of type "text/x-patch" (11939 bytes)

View attachment "0002-locking-pvqspinlock-Introduce-CONFIG_PARAVIRT_QSPINL.patch" of type "text/x-patch" (6167 bytes)

View attachment "0009-powerpc-pseries-Fixup.patch" of type "text/x-patch" (3088 bytes)