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

[Mateusz Guzik]

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.