linux-kernel - Re: [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before

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Message-Id: <1607918323.6muyu2l982.astroid@bobo.none>
Date:   Mon, 14 Dec 2020 14:07:27 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Anton Blanchard <anton@...abs.org>, Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 2/8] x86: use exit_lazy_tlb rather than
 membarrier_mm_sync_core_before_usermode

Excerpts from Andy Lutomirski's message of December 11, 2020 10:11 am:
>> On Dec 5, 2020, at 7:59 PM, Nicholas Piggin <npiggin@...il.com> wrote:
>>
> 
>> I'm still going to persue shoot-lazies for the merge window. As you
>> see it's about a dozen lines and a if (IS_ENABLED(... in core code.
>> Your change is common code, but a significant complexity (which
>> affects all archs) so needs a lot more review and testing at this
>> point.
> 
> I don't think it's ready for this merge window.

Yes next one I meant (aka this one for development perspective :)).

> I read the early
> patches again, and I think they make the membarrier code worse, not
> better.

Mathieu and I disagree, so we are at an impasse. I addressed your 
comment about not being able to do the additional core sync avoidance 
from the exit tlb call (you can indeed do so in your arch code) and 
about exit_lazy_tlb being a call into the scheduler (it's not) and
about the arch code not being able to reconcile lazy tlb mm with the
core scheduler code (you can).

I fundamentally think the core sync is an issue with what the membarrier
/ arch specifics are doing with lazy tlb mm switching, and not something
the core scheduler needs to know about at all. I don't see the big
problem with essentially moving it from an explicit call to 
exit_lazy_tlb (which from scheduler POV describes better what it is 
doing, not how).

> I'm not fundamentally opposed to the shoot-lazies concept,
> but it needs more thought and it needs a cleaner foundation.

Well shoot lazies actually doesn't really rely on that membarrier
change at all, it just came as a nice looking cleanup so that part
can be dropped from the series. It's not really foundational.

Thanks,
Nick