linux-kernel - Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm refcounting

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Date:   Mon, 30 Jul 2018 15:36:08 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>, X86 ML <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Dave Hansen <dave.hansen@...el.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm
 refcounting

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@...riel.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?

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.

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