linux-kernel - Re: [RFC PATCH v2 3/3] x86/mm/tlb: Avoid deferring PTI flushes on shootdown

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Date:   Tue, 27 Aug 2019 16:07:55 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH v2 3/3] x86/mm/tlb: Avoid deferring PTI flushes on shootdown

On Fri, Aug 23, 2019 at 11:13 PM Nadav Amit <namit@...are.com> wrote:
>
> When a shootdown is initiated, the initiating CPU has cycles to burn as
> it waits for the responding CPUs to receive the IPI and acknowledge it.
> In these cycles it is better to flush the user page-tables using
> INVPCID, instead of deferring the TLB flush.
>
> The best way to figure out whether there are cycles to burn is arguably
> to expose from the SMP layer when an acknowledgment is received.
> However, this would break some abstractions.
>
> Instead, use a simpler solution: the initiating CPU of a TLB shootdown
> would not defer PTI flushes. It is not always a win, relatively to
> deferring user page-table flushes, but it prevents performance
> regression.
>
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
>  arch/x86/include/asm/tlbflush.h |  1 +
>  arch/x86/mm/tlb.c               | 10 +++++++++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index da56aa3ccd07..066b3804f876 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -573,6 +573,7 @@ struct flush_tlb_info {
>         unsigned int            initiating_cpu;
>         u8                      stride_shift;
>         u8                      freed_tables;
> +       u8                      shootdown;

I find the name "shootdown" to be confusing.  How about "more_than_one_cpu"?