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

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Message-ID: <CAJuCfpEe9Sas46r1QUEe7Bu3QD5atKUxy3E5Z8Xg66aZS81suQ@mail.gmail.com>
Date: Wed, 13 Nov 2024 07:45:21 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 7:02 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> 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?
>
> 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 :)

Yeah, I had trouble wording all the reasons because in my head it was
simply "the right thing to do". Now with all your input my job has
become much easier :) Thanks folks!