linux-kernel - Re: [RFC PATCH v2 08/12] x86/tlb: Privatize cpu

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Message-ID: <63172D88-998F-43F1-AB6F-F4A13B90AD9D@vmware.com>
Date:   Fri, 31 May 2019 19:42:42 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...capital.net>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [RFC PATCH v2 08/12] x86/tlb: Privatize cpu_tlbstate

> On May 31, 2019, at 11:48 AM, Andy Lutomirski <luto@...capital.net> wrote:
> 
> 
>> On May 30, 2019, at 11:36 PM, Nadav Amit <namit@...are.com> wrote:
>> 
>> cpu_tlbstate is mostly private and only the variable is_lazy is shared.
>> This causes some false-sharing when TLB flushes are performed.
>> 
>> Break cpu_tlbstate intro cpu_tlbstate and cpu_tlbstate_shared, and mark
>> each one accordingly.
> 
> At some point we’ll want to do a better job with our PV flush code, and I
> suspect we’ll end up with more of this shared again.

In the usual use-case, when you flush the TLB, will you write something to
cpu_tlbstate that should be visible to other cores? I don’t see why, even if
PV is used.

Anyhow, I was always under the impression that PV should not punish
bare-metal.

The other option is to take cpu_tlbstate and rearrange it so the shared
stuff will not be next to the private. I just don’t know how to do it
without making an assumption of the cacheline size.