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

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Date: Tue, 6 Feb 2024 20:47:27 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
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, Feb 06, 2024, David Woodhouse wrote:
> On Tue, 2024-02-06 at 20:22 -0800, Sean Christopherson wrote:
> > On Mon, Jan 15, 2024, Paul Durrant wrote:
> > > From: Paul Durrant <pdurrant@...zon.com>
> > > 
> > > Taking a write lock on a pfncache will be disruptive if the cache is
> > 
> > *Unnecessarily* taking a write lock.
> 
> No. Taking a write lock will be disrupting.
> 
> Unnecessarily taking a write lock will be unnecessarily disrupting.
> 
> Taking a write lock on a Thursday will be disrupting on a Thursday.
> 
> But the key is that if the cache is heavily used, the user gets
> disrupted.

If the invalidation is relevant, then this code is taking gpc->lock for write no
matter what.  The purpose of the changelog is to explain _why_ a patch adds value.

> >   Please save readers a bit of brain power
> > and explain that this is beneificial when there are _unrelated_ invalidation.
> 
> I don't understand what you're saying there. Paul's sentence did have
> an implicit "...so do that less then", but that didn't take much brain
> power to infer.

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).

Is it too much hand-holding, and bordering on stating the obvious?  Maybe.  But
(a) a lot of people that read mailing lists and KVM code are *not* kernel experts,
and (b) a changelog is written _once_, and read hundreds if not thousands of times.

If we can save each reader even a few seconds, then taking an extra minute or two
to write a more verbose changelog is a net win.