Re: [PATCH v2 2/5] mm: move per-vma lock into vm_area_struct

[Lorenzo Stoakes]

On Wed, Nov 13, 2024 at 03:53:54PM +0100, Mateusz Guzik wrote:
> On Wed, Nov 13, 2024 at 02:28:16PM +0000, Lorenzo Stoakes wrote:
> > On Tue, Nov 12, 2024 at 11:46:32AM -0800, Suren Baghdasaryan wrote:
> > > Back when per-vma locks were introduces, vm_lock was moved out of
> > > vm_area_struct in [1] because of the performance regression caused by
> > > false cacheline sharing. Recent investigation [2] revealed that the
> > > regressions is limited to a rather old Broadwell microarchitecture and
> > > even there it can be mitigated by disabling adjacent cacheline
> > > prefetching, see [3].
> >
> > I don't see a motivating reason as to why we want to do this? We increase
> > memory usage here which is not good, but later lock optimisation mitigates
> > it, but why wouldn't we just do the lock optimisations and use less memory
> > overall?
> >
>
> Where would you put the lock in that case though?
>
> With the patchset it sticks with the affected vma, so no false-sharing
> woes concerning other instances of the same struct.
>
> If you make them separately allocated and packed, they false-share
> between different vmas using them (in fact this is currently happening).
> If you make sure to pad them, that's 64 bytes per obj, majority of which
> is empty space.

'I don't see a motivating reason' = I don't see it in the commit message.

I'm saying put motivating reasons, like the above, in the commit message.