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

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Message-ID: <877dzf4a8v.fsf@nanos.tec.linutronix.de>
Date:   Fri, 20 Mar 2020 11:00:00 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Kyung Min Park <kyung.min.park@...el.com>
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>,
        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 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.

> That will also make it a lot easier to understand the oddity with
> start and cycles.

Indeed. That makes sense. Should have thought about it :)

Thanks,

        tglx