linux-kernel - Re: [PATCH v12 18/20] KVM: pfncache: check the need for invalidation under read lock first

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Message-ID: <a817d64f3fe7b935a02e78df02dc0c6281e61af3.camel@infradead.org>
Date: Tue, 06 Feb 2024 20:59:26 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paul Durrant <paul@....org>, Paolo Bonzini <pbonzini@...hat.com>, 
 Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin"
 <hpa@...or.com>,  Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org,
 linux-doc@...r.kernel.org,  linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v12 18/20] KVM: pfncache: check the need for
 invalidation under read lock first

On Tue, 2024-02-06 at 20:47 -0800, Sean Christopherson wrote:
> 
> I'm saying this:
> 
>   When processing mmu_notifier invalidations for gpc caches, pre-check for
>   overlap with the invalidation event while holding gpc->lock for read, and
>   only take gpc->lock for write if the cache needs to be invalidated  Doing
>   a pre-check without taking gpc->lock for write avoids unnecessarily
>   contending the lock for unrelated invalidations, which is very beneficial
>   for caches that are heavily used (but rarely subjected to mmu_notifier
>   invalidations).
> 
> is much friendlier to readers than this:
> 
>   Taking a write lock on a pfncache will be disruptive if the cache is
>   heavily used (which only requires a read lock). Hence, in the MMU notifier
>   callback, take read locks on caches to check for a match; only taking a
>   write lock to actually perform an invalidation (after a another check).

That's a somewhat subjective observation. I actually find the latter to
be far more succinct and obvious.

Actually... maybe I find yours harder because it isn't actually stating
the situation as I understand it. You said "unrelated invalidation" in
your first email, and "overlap with the invalidation event" in this
one... neither of which makes sense to me because there is no *other*
invalidation here.

We're only talking about the MMU notifier gratuitously taking the write
lock on a GPC that it *isn't* going to invalidate (the common case),
and that disrupting users which are trying to take the read lock on
that GPC.

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