linux-kernel - Re: [PATCH v4 0/5] move per-vma lock into vm_area

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Message-ID: <CAJuCfpGjm92OiWO_APpZRsRdgC_a=g5cTmwfHoovOPYd+-RGPQ@mail.gmail.com>
Date: Wed, 20 Nov 2024 15:52:59 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
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	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, souravpanda@...gle.com, pasha.tatashin@...een.com, 
	corbet@....net, linux-doc@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v4 0/5] move per-vma lock into vm_area_struct

On Wed, Nov 20, 2024 at 2:10 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> On Tue, Nov 19, 2024 at 04:08:21PM -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].
>
> If 'struct vm_area_struct' is prone to performance issues due to
> cacheline misalignments then we should do something about the
> __randomize_layout tag for it. I imagine we can identify the fields
> which might be performance critical to be on same cacheline or different
> cacheline due to false sharing then we can divide the fields into
> different cacheline groups and fields can be __randomize_layout within
> the group. WDYT?

I think that's a good idea since shuffling these fields around does
affect performance. I can look into it once these changes get
finalized and the layout gets more stable.

>