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

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Message-ID: <CALCETrWFprNc2V1nrg_y0RDhV_PkSnx6v_p=mUHvpv9E0C5zAw@mail.gmail.com>
Date:   Tue, 27 Aug 2019 17:30:15 -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 Tue, Aug 27, 2019 at 4:57 PM Nadav Amit <namit@...are.com> wrote:
>
> > On Aug 27, 2019, at 4:07 PM, Andy Lutomirski <luto@...nel.org> wrote:
> >
> > 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”?
>
> I think the current semantic is more of “includes remote cpus”. How about
> calling it “local_only”, and negating its value?

Sure.