linux-kernel - Re: [RFC PATCH 1/5] x86: tsc: add tsc to art helpers

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Message-ID: <alpine.DEB.2.21.1908151458560.1923@nanos.tec.linutronix.de>
Date:   Thu, 15 Aug 2019 16:16:09 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>
cc:     Richard Cochran <richardcochran@...il.com>, netdev@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Christopher S . Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH 1/5] x86: tsc: add tsc to art helpers

Felipe,

On Thu, 15 Aug 2019, Felipe Balbi wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
> > On Tue, 16 Jul 2019, Felipe Balbi wrote:
> >
> > So some information what those interfaces are used for and why they are
> > needed would be really helpful.
> 
> Okay, I have some more details about this. The TGPIO device itself uses
> ART since TSC is not directly available to anything other than the
> CPU. The 'problem' here is that reading ART incurs extra latency which
> we would like to avoid. Therefore, we use TSC and scale it to
> nanoseconds which, would be the same as ART to ns.

Fine. But that's not really correct:

      TSC = art_to_tsc_offset + ART * scale;
 
> >> +void get_tsc_ns(struct system_counterval_t *tsc_counterval, u64 *tsc_ns)

Why is this not returning the result instead of having that pointer
indirection?

> >> +{
> >> +	u64 tmp, res, rem;
> >> +	u64 cycles;
> >> +
> >> +	tsc_counterval->cycles = clocksource_tsc.read(NULL);
> >> +	cycles = tsc_counterval->cycles;
> >> +	tsc_counterval->cs = art_related_clocksource;

So this does more than returning the TSC time converted to nanoseconds. The
function name should reflect this. Plus both functions want kernel-doc
explaining what they do.

> >> +	rem = do_div(cycles, tsc_khz);
> >> +
> >> +	res = cycles * USEC_PER_SEC;
> >> +	tmp = rem * USEC_PER_SEC;
> >> +
> >> +	do_div(tmp, tsc_khz);
> >> +	res += tmp;
> >> +
> >> +	*tsc_ns = res;
> >> +}
> >> +EXPORT_SYMBOL(get_tsc_ns);
> >> +
> >> +u64 get_art_ns_now(void)
> >> +{
> >> +	struct system_counterval_t tsc_cycles;
> >> +	u64 tsc_ns;
> >> +
> >> +	get_tsc_ns(&tsc_cycles, &tsc_ns);
> >> +
> >> +	return tsc_ns;
> >> +}
> >> +EXPORT_SYMBOL(get_art_ns_now);
> >
> > While the changes look innocuous I'm missing the big picture why this needs
> > to emulate ART instead of simply using TSC directly.
> 
> i don't think we're emulating ART here (other than the name in the
> function). We're just reading TSC and converting to nanoseconds, right?

Well, the function name says clearly: get_art_ns_now(). But you are not
using ART, you use the TSC and derive the ART value from it (incorrectly).

Thanks,

	tglx