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

From: Andy Lutomirski
Date: Mon Jun 19 2017 - 18:03:01 EST

On Sat, Jun 17, 2017 at 11:26 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>> On Jun 13, 2017, at 9:56 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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 Is this commit message outdated?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.