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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 30 Jul 2018 12:49:12 -0700
Message-ID: <CALCETrXnG9nPzJKOtd=65BWeoTs2y=mVUAEvNGmFrD8fvg-EfQ@mail.gmail.com>
Subject: Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm refcounting
To:     Rik van Riel <riel@surriel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>, X86 ML <x86@kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Mike Galbraith <efault@gmx.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
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On Mon, Jul 30, 2018 at 12:36 PM, Rik van Riel <riel@surriel.com> wrote:
> On Mon, 2018-07-30 at 12:30 -0700, Andy Lutomirski wrote:
>> On Mon, Jul 30, 2018 at 12:15 PM, Rik van Riel <riel@surriel.com>
>> wrote:
>> > On Mon, 2018-07-30 at 18:26 +0200, Peter Zijlstra wrote:
>> > > On Mon, Jul 30, 2018 at 10:30:11AM -0400, Rik van Riel wrote:
>> > >
>> > > > > What happened to the rework I did there? That not only
>> > > > > avoided
>> > > > > fiddling
>> > > > > with active_mm, but also avoids grab/drop cycles for the
>> > > > > other
>> > > > > architectures when doing task->kthread->kthread->task things.
>> > > >
>> > > > I don't think I saw that. I only saw your email from
>> > > > July 20th with this fragment of code, which does not
>> > > > appear to avoid the grab/drop cycles, and still fiddles
>> > > > with active_mm:
>> > >
>> > > Yeah, that's it. Note how it doesn't do a grab+drop for kernel-
>> > > > kernel,
>> > >
>> > > where the current could would have.
>> > >
>> > > And also note that it only fiddles with active_mm if it does the
>> > > grab+drop thing (the below should have s/ifdef/ifndef/ to make
>> > > more
>> > > sense maybe).
>> >
>> > I'll kick off a test with your variant. I don't think we
>> > will see any performance difference on x86 (due to not
>> > using a refcount at all any more), but unless Ingo is in
>> > a hurry I guess there's no issue rewriting this part of
>> > the patch series :)
>> >
>> > Do the other patches look ok to you and Andy?
>> >
>>
>> The whole series other than the active_mm stuff looked okay to me.
>
> Does the active_mm stuff look like a step in the right
> direction with the bugfix, or would you prefer the code
> to go in an entirely different direction?

I think it's a big step in the right direction, but it still makes be
nervous.  I'd be more comfortable with it if you at least had a
functional set of patches that result in active_mm being gone, because
that will mean that you actually audited the whole mess and fixed
anything that might rely on active_mm pointing somewhere or that might
be putting a value you didn't take into account into active_mm.  IOW
I'm not totally thrilled by applying the patches as is if we're still
a bit unsure as to what might have gotten missed.

I don't think it's at all necessary to redo the patches.

Does that seem reasonable?

>
> If this looks like a step in the right direction, it
> may make sense to make this step before the merge window
> opens, and continue with more patches in this direction
> later.
>
> --
> All Rights Reversed.