linux-kernel - Re: [PATCH v4 10/10] x86/mm: Try to preserve old TLB entries using PCID

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Date:   Wed, 5 Jul 2017 19:02:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
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>,
        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>
Subject: Re: [PATCH v4 10/10] x86/mm: Try to preserve old TLB entries using
 PCID

On Wed, Jul 05, 2017 at 09:04:39AM -0700, Andy Lutomirski wrote:
> On Wed, Jul 5, 2017 at 5:18 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Thu, Jun 29, 2017 at 08:53:22AM -0700, Andy Lutomirski wrote:
> >> @@ -104,18 +140,20 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> >>
> >>               /* Resume remote flushes and then read tlb_gen. */
> >>               cpumask_set_cpu(cpu, mm_cpumask(next));
> >
> > Barriers should have a comment... what is being ordered here against
> > what?
> 
> How's this comment?
> 
>         /*
>          * Resume remote flushes and then read tlb_gen.  We need to do
>          * it in this order: any inc_mm_tlb_gen() caller that writes a
>          * larger tlb_gen than we read here must see our cpu set in
>          * mm_cpumask() so that it will know to flush us.  The barrier
>          * here synchronizes with inc_mm_tlb_gen().
>          */

Slightly confusing, you mean this, right?


	cpumask_set_cpu(cpu, mm_cpumask());			inc_mm_tlb_gen();

	MB							MB

	next_tlb_gen = atomic64_read(&next->context.tlb_gen);	flush_tlb_others(mm_cpumask());


which seems to make sense.