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

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Date:   Wed, 26 Jun 2019 01:22:38 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     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>,
        the arch/x86 maintainers <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 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.

Sounds good. I will add another patch based on your idea.