linux-kernel - Re: [PATCH v3 5/9] x86/mm/tlb: Privatize cpu

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Message-ID: <FCEC2890-40B8-4114-913E-7C05B021C4EA@vmware.com>
Date:   Fri, 19 Jul 2019 18:43:30 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v3 5/9] x86/mm/tlb: Privatize cpu_tlbstate

> On Jul 19, 2019, at 11:38 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 7/18/19 5:58 PM, Nadav Amit wrote:
>> +struct tlb_state_shared {
>> +	/*
>> +	 * We can be in one of several states:
>> +	 *
>> +	 *  - Actively using an mm.  Our CPU's bit will be set in
>> +	 *    mm_cpumask(loaded_mm) and is_lazy == false;
>> +	 *
>> +	 *  - Not using a real mm.  loaded_mm == &init_mm.  Our CPU's bit
>> +	 *    will not be set in mm_cpumask(&init_mm) and is_lazy == false.
>> +	 *
>> +	 *  - Lazily using a real mm.  loaded_mm != &init_mm, our bit
>> +	 *    is set in mm_cpumask(loaded_mm), but is_lazy == true.
>> +	 *    We're heuristically guessing that the CR3 load we
>> +	 *    skipped more than makes up for the overhead added by
>> +	 *    lazy mode.
>> +	 */
>> +	bool is_lazy;
>> +};
>> +DECLARE_PER_CPU_SHARED_ALIGNED(struct tlb_state_shared, cpu_tlbstate_shared);
> 
> Could we get a comment about what "shared" means and why we need shared
> state?
> 
> Should we change 'tlb_state' to 'tlb_state_private’?

Andy said that for the lazy tlb optimizations there might soon be more
shared state. If you prefer, I can move is_lazy outside of tlb_state, and
not set it in any alternative struct.