linux-kernel - Re: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay

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Date:   Fri, 20 Mar 2020 16:57:03 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kyung Min Park <kyung.min.park@...el.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay

On Fri, Mar 20, 2020 at 4:23 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Andy Lutomirski <luto@...nel.org> writes:
>
> > On Fri, Mar 20, 2020 at 3:00 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >>
> >> Andy Lutomirski <luto@...nel.org> writes:
> >> > On Thu, Mar 19, 2020 at 9:13 PM Kyung Min Park <kyung.min.park@...el.com> wrote:
> >> >>  void use_tsc_delay(void)
> >> >>  {
> >> >> -       if (delay_fn == delay_loop)
> >> >> +       if (static_cpu_has(X86_FEATURE_WAITPKG)) {
> >> >> +               delay_halt_fn = delay_halt_tpause;
> >> >> +               delay_fn = delay_halt;
> >> >> +       } else if (delay_fn == delay_loop) {
> >> >>                 delay_fn = delay_tsc;
> >> >> +       }
> >> >>  }
> >> >
> >> > This is an odd way to dispatch: you're using static_cpu_has(), but
> >> > you're using it once to populate a function pointer.  Why not just put
> >> > the static_cpu_has() directly into delay_halt() and open-code the
> >> > three variants?
> >>
> >> Two: mwaitx and tpause.
> >
> > I was imagining there would also be a variant for systems with neither feature.
>
> Oh I see, you want to get rid of both function pointers. That's tricky.
>
> The boot time function is delay_loop() which is using the magic (1 << 12)
> boot time value until calibration in one way or the other happens and
> something calls use_tsc_delay() or use_mwaitx_delay(). Yes, that's all
> horrible but X86_FEATURE_TSC is unusable for this.
>
> Let me think about it.

This is definitely not worth overoptimizing.  It's a *delay* function
-- the retpoline isn't going to kill us :)

>
> Thanks,
>
>         tglx
>
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