linux-kernel - Re: [PATCH v3 06/11] x86/mm: Rework lazy TLB mode and TLB freshness tracking

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Date:   Thu, 27 Jul 2017 19:05:46 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andrew Banman <abanman@....com>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Banman <abanman@....com>, Mike Travis <travis@....com>,
        Dimitri Sivanich <sivanich@....com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v3 06/11] x86/mm: Rework lazy TLB mode and TLB freshness tracking

> On Jul 27, 2017, at 3:53 PM, Andrew Banman <abanman@....com> wrote:
>
>> On Thu, Jun 22, 2017 at 10:47:29AM -0700, Andy Lutomirski wrote:
>>> On Thu, Jun 22, 2017 at 7:50 AM, Borislav Petkov <bp@...en8.de> wrote:
>>>> On Tue, Jun 20, 2017 at 10:22:12PM -0700, Andy Lutomirski wrote:
>>>> Rewrite it entirely.  When we enter lazy mode, we simply remove the
>>>> cpu from mm_cpumask.  This means that we need a way to figure out
>>>
>>> s/cpu/CPU/
>>
>> Done.
>>
>>>
>>>> whether we've missed a flush when we switch back out of lazy mode.
>>>> I use the tlb_gen machinery to track whether a context is up to
>>>> date.
>>>>
>>>> Note to reviewers: this patch, my itself, looks a bit odd.  I'm
>>>> using an array of length 1 containing (ctx_id, tlb_gen) rather than
>>>> just storing tlb_gen, and making it at array isn't necessary yet.
>>>> I'm doing this because the next few patches add PCID support, and,
>>>> with PCID, we need ctx_id, and the array will end up with a length
>>>> greater than 1.  Making it an array now means that there will be
>>>> less churn and therefore less stress on your eyeballs.
>>>>
>>>> NB: This is dubious but, AFAICT, still correct on Xen and UV.
>>>> xen_exit_mmap() uses mm_cpumask() for nefarious purposes and this
>>>> patch changes the way that mm_cpumask() works.  This should be okay,
>>>> since Xen *also* iterates all online CPUs to find all the CPUs it
>>>> needs to twiddle.
>>>
>>> This whole text should be under the "---" line below if we don't want it
>>> in the commit message.
>>
>> I figured that some future reader of this patch might actually want to
>> see this text, though.
>>
>>>
>>>>
>>>> The UV tlbflush code is rather dated and should be changed.
>>
>> And I'd definitely like the UV maintainers to notice this part, now or
>> in the future :)  I don't want to personally touch the UV code with a
>> ten-foot pole, but it really should be updated by someone who has a
>> chance of getting it right and being able to test it.
>
> Noticed! We're aware of these changes and we're planning on updating this
> code in the future. Presently the BAU tlb shootdown feature is working well
> on our recent hardware.

:)

I would suggest reworking it to hook the SMP function call
infrastructure instead of the TLB shootdown code.