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

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Date:   Sat, 17 Jun 2017 23:26:48 -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,
        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 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Is this commit message outdated? NR_DYNAMIC_ASIDS is set to 6.
More importantly, I do not see PCID 0 as reserved:

> +static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
> +			    u16 *new_asid, bool *need_flush)
> +{
> 

[snip]

> +	if (*new_asid >= NR_DYNAMIC_ASIDS) {
> +		*new_asid = 0;
> +		this_cpu_write(cpu_tlbstate.next_asid, 1);
> +	}
> +	*need_flush = true;
> +}


Am I missing something?