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

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Message-ID: <5d34da83-92eb-4666-816c-73a0e831aa89@lucifer.local>
Date: Wed, 13 Nov 2024 15:01:48 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com,
        mhocko@...e.com, vbabka@...e.cz, hannes@...xchg.org, mjguzik@...il.com,
        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 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?

I worded this badly. To clarify:

I don't see a motivating reason _in the commit message_ as to why we want
to do this.

I am certain there are, in fact Mateusz and Vlastimil have provided them.

So my review is - let's just put these there :)