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

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Date:   Tue, 18 Jul 2017 10:06:15 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        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 Tue, Jul 18, 2017 at 1:53 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Peter Zijlstra <peterz@...radead.org> wrote:
>
>> 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.
>
> Btw., I'll wait for a v5 iteration before applying this last patch to tip:x86/mm.

I'll send it shortly.  I think I'll also add a patch to factor out the
flush calls a bit more to prepare for Mel's upcoming fix.

>
> Thanks,
>
>         Ingo