linux-kernel - Re: [PATCH] x86/apic: Handle missing global clockevent gracefully

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Message-ID: <CAERHkrttXdZhZHZs+JasZU6a2kEb1vc6KB25+LbpQycenJZpOg@mail.gmail.com>
Date:   Mon, 12 Aug 2019 20:59:42 +0800
From:   Aubrey Li <aubrey.intel@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Daniel Drake <drake@...lessm.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Endless Linux Upstreaming Team <linux@...lessm.com>,
        Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH] x86/apic: Handle missing global clockevent gracefully

On Mon, Aug 12, 2019 at 8:25 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Mon, 12 Aug 2019, Li, Aubrey wrote:
> > On 2019/8/9 20:54, Thomas Gleixner wrote:
> > > +   local_irq_disable();
> > >     /*
> > >      * Setup the APIC counter to maximum. There is no way the lapic
> > >      * can underflow in the 100ms detection time frame
> > >      */
> > >     __setup_APIC_LVTT(0xffffffff, 0, 0);
> > >
> > > -   /* Let the interrupts run */
> > > -   local_irq_enable();
> > > +   /*
> > > +    * Methods to terminate the calibration loop:
> > > +    *  1) Global clockevent if available (jiffies)
> > > +    *  2) TSC if available and frequency is known
> > > +    */
> > > +   jif_start = READ_ONCE(jiffies);
> > > +
> > > +   if (tsc_khz) {
> > > +           tsc_start = rdtsc();
> > > +           tsc_perj = div_u64((u64)tsc_khz * 1000, HZ);
> > > +   }
> > > +
> > > +   while (lapic_cal_loops <= LAPIC_CAL_LOOPS) {
> >
> > Is this loop still meaningful, can we just invoke the handler twice
> > before and after the tick?
>
> And that solves what?
>

I meant, can we do this one time?
- lapic_cal_t1 = read APIC counter
- /* Wait for a tick to elapse */
- lapic_cal_t2 = read APIC counter

I'm not clear why we still need this loop, to use the
existing lapic_cal_handler()?

Thanks,
-Aubrey