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

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On Tue, 2024-02-06 at 20:22 -0800, Sean Christopherson wrote:
> On Mon, Jan 15, 2024, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> > 
> > 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.


>   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.

> > 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).
> 
> This doesn't have any dependency on this series, does it?  I.e. this should be
> posted separately, and preferably with some performance data.  Not having data
> isn't a sticking point, but it would be nice to verify that this isn't a
> pointless optimization.

No fundamental dependency, no. But it was triggered by the previous
patch, which makes kvm_xen_set_evtchn_fast() use read_trylock() and
makes it take the slow path when there's contention. It lives here just
fine as part of the series.

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