linux-kernel - Re: [PATCH v2 10/10] x86/mm: Try to preserve old TLB entries using PCID

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Date:   Mon, 19 Jun 2017 15:02:33 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <nadav.amit@...il.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>,
        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>
Subject: Re: [PATCH v2 10/10] x86/mm: Try to preserve old TLB entries using PCID

On Sat, Jun 17, 2017 at 11:26 PM, Nadav Amit <nadav.amit@...il.com> wrote:
>
>> On Jun 13, 2017, at 9:56 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>
>> PCID is a "process context ID" -- it's what other architectures call
>> an address space ID.  Every non-global TLB entry is tagged with a
>> PCID, only TLB entries that match the currently selected PCID are
>> used, and we can switch PGDs without flushing the TLB.  x86's
>> PCID is 12 bits.
>>
>> This is an unorthodox approach to using PCID.  x86's PCID is far too
>> short to uniquely identify a process, and we can't even really
>> uniquely identify a running process because there are monster
>> systems with over 4096 CPUs.  To make matters worse, past attempts
>> to use all 12 PCID bits have resulted in slowdowns instead of
>> speedups.
>>
>> This patch uses PCID differently.  We use a PCID to identify a
>> recently-used mm on a per-cpu basis.  An mm has no fixed PCID
>> binding at all; instead, we give it a fresh PCID each time it's
>> loaded except in cases where we want to preserve the TLB, in which
>> case we reuse a recent value.
>>
>> In particular, we use PCIDs 1-3 for recently-used mms and we reserve
>> PCID 0 for swapper_pg_dir and for PCID-unaware CR3 users (e.g. EFI).
>> Nothing ever switches to PCID 0 without flushing PCID 0 non-global
>> pages, so PCID 0 conflicts won't cause problems.
>
> Is this commit message outdated?

Yes, it's old.  Will fix.