linux-kernel - Re: [PATCH v2 05/10] x86/mm: Rework lazy TLB mode and TLB freshness tracking

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Date:   Mon, 19 Jun 2017 15:00:51 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 05/10] x86/mm: Rework lazy TLB mode and TLB freshness tracking

On Tue, Jun 13, 2017 at 11:09 PM, Juergen Gross <jgross@...e.com> wrote:
> On 14/06/17 06:56, Andy Lutomirski wrote:
>> x86's lazy TLB mode used to be fairly weak -- it would switch to
>> init_mm the first time it tried to flush a lazy TLB.  This meant an
>> unnecessary CR3 write and, if the flush was remote, an unnecessary
>> IPI.
>>
>> 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
>> 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.
>
> There is a allocation failure path in xen_drop_mm_ref() which might
> be wrong with this patch. As this path should be taken only very
> unlikely I'd suggest to remove the test for mm_cpumask() bit zero in
> this path.
>

Right, fixed.

>
> Juergen