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

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Message-Id: <29636D5E-53D4-47B1-8F72-8DD0FAE58A60@gmail.com>
Date:   Mon, 19 Jun 2017 15:53:20 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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

Andy Lutomirski <luto@...nel.org> wrote:

> 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 Is this commit message outdated?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.

Just to clarify: I asked since I don’t understand how the interaction with
PCID-unaware CR3 users go. Specifically, IIUC, arch_efi_call_virt_teardown()
can reload CR3 with an old PCID value. No?