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

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Message-ID: <b8a8bf77-da29-4fd1-8def-437e23c35601@lucifer.local>
Date: Wed, 13 Nov 2024 14:59:30 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        willy@...radead.org, liam.howlett@...cle.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, oliver.sang@...el.com,
        mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com,
        oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org,
        brauner@...nel.org, dhowells@...hat.com, hdanton@...a.com,
        hughd@...gle.com, minchan@...gle.com, jannh@...gle.com,
        shakeel.butt@...ux.dev, souravpanda@...gle.com,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 2/5] mm: move per-vma lock into vm_area_struct

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.