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Date:   Wed, 16 Jun 2021 19:57:57 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Nicholas Piggin" <npiggin@gmail.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc:     "Andrew Morton" <akpm@linux-foundation.org>,
        "Dave Hansen" <dave.hansen@intel.com>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        linux-mm@kvack.org,
        "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
        "the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit
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On Wed, Jun 16, 2021, at 6:37 PM, Nicholas Piggin wrote:
> Excerpts from Andy Lutomirski's message of June 17, 2021 4:41 am:
> > On 6/16/21 12:35 AM, Peter Zijlstra wrote:
> >> On Wed, Jun 16, 2021 at 02:19:49PM +1000, Nicholas Piggin wrote:
> >>> Excerpts from Andy Lutomirski's message of June 16, 2021 1:21 pm:
> >>>> membarrier() needs a barrier after any CPU changes mm.  There is =
currently
> >>>> a comment explaining why this barrier probably exists in all case=
s.  This
> >>>> is very fragile -- any change to the relevant parts of the schedu=
ler
> >>>> might get rid of these barriers, and it's not really clear to me =
that
> >>>> the barrier actually exists in all necessary cases.
> >>>
> >>> The comments and barriers in the mmdrop() hunks? I don't see what =
is=20
> >>> fragile or maybe-buggy about this. The barrier definitely exists.
> >>>
> >>> And any change can change anything, that doesn't make it fragile. =
My
> >>> lazy tlb refcounting change avoids the mmdrop in some cases, but i=
t
> >>> replaces it with smp_mb for example.
> >>=20
> >> I'm with Nick again, on this. You're adding extra barriers for no
> >> discernible reason, that's not generally encouraged, seeing how ext=
ra
> >> barriers is extra slow.
> >>=20
> >> Both mmdrop() itself, as well as the callsite have comments saying =
how
> >> membarrier relies on the implied barrier, what's fragile about that=
?
> >>=20
> >=20
> > My real motivation is that mmgrab() and mmdrop() don't actually need=
 to
> > be full barriers.  The current implementation has them being full
> > barriers, and the current implementation is quite slow.  So let's tr=
y
> > that commit message again:
> >=20
> > membarrier() needs a barrier after any CPU changes mm.  There is cur=
rently
> > a comment explaining why this barrier probably exists in all cases. =
The
> > logic is based on ensuring that the barrier exists on every control =
flow
> > path through the scheduler.  It also relies on mmgrab() and mmdrop()=
 being
> > full barriers.
> >=20
> > mmgrab() and mmdrop() would be better if they were not full barriers=
.  As a
> > trivial optimization, mmgrab() could use a relaxed atomic and mmdrop=
()
> > could use a release on architectures that have these operations.
>=20
> I'm not against the idea, I've looked at something similar before (not=

> for mmdrop but a different primitive). Also my lazy tlb shootdown seri=
es=20
> could possibly take advantage of this, I might cherry pick it and test=
=20
> performance :)
>=20
> I don't think it belongs in this series though. Should go together wit=
h
> something that takes advantage of it.

I=E2=80=99m going to see if I can get hazard pointers into shape quickly=
.

>=20
> Thanks,
> Nick
>=20