linux-kernel - Re: [PATCH 8/9] x86/tlb: Privatize cpu

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Date:   Tue, 25 Jun 2019 20:57:54 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 8/9] x86/tlb: Privatize cpu_tlbstate

On Tue, Jun 25, 2019 at 2:52 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 6/12/19 11:48 PM, Nadav Amit wrote:
> > cpu_tlbstate is mostly private and only the variable is_lazy is shared.
> > This causes some false-sharing when TLB flushes are performed.
>
> Presumably, all CPUs doing TLB flushes read 'is_lazy'.  Because of this,
> when we write to it we have to do the cache coherency dance to get rid
> of all the CPUs that might have a read-only copy.
>
> I would have *thought* that we only do writes when we enter or exist
> lazy mode.  That's partially true.  We do write in enter_lazy_tlb(), but
> we also *unconditionally* write in switch_mm_irqs_off().  That seems
> like it might be responsible for a chunk (or even a vast majority) of
> the cacheline bounces.
>
> Is there anything preventing us from turning the switch_mm_irqs_off()
> write into:
>
>         if (was_lazy)
>                 this_cpu_write(cpu_tlbstate.is_lazy, false);
>
> ?
>
> I think this patch is probably still a good general idea, but I just
> wonder if reducing the writes is a better way to reduce bounces.

Good catch!  I'm usually pretty good about this for
test_and_set()-style things, but I totally missed this obvious
unnecessary write when I did this.  I hereby apologize for all the
cycles I wasted :)

--Andy