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

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Date:   Mon, 30 Mar 2020 23:42:37 +0000
From:   "Park, Kyung Min" <kyung.min.park@...el.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     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>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>
Subject: RE: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay

Hi Andy/Thomas,

 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 :)

Since the use_tsc_delay() is used just once in __init tsc_init(), 
how about adding "__init" to the use_tsc_delay() and keep these function pointers?