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

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Message-ID: <87zhcaobjt.fsf@nanos.tec.linutronix.de>
Date:   Sat, 21 Mar 2020 00:23:50 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
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

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.

Thanks,

        tglx