linux-kernel - Re: [PATCH v2] timekeeping: get_jiffies_boot_64() for jiffies that include sleep time

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Date:   Wed, 19 Jun 2019 22:02:07 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] timekeeping: get_jiffies_boot_64() for jiffies that
 include sleep time

On Wed, Jun 19, 2019 at 5:31 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Hi Arnd,
>
> On Wed, Jun 19, 2019 at 5:08 PM Arnd Bergmann <arnd@...db.de> wrote:
> > Can you quantify how much this gains you over ktime_get_coarse_boottime
> > in practice? You are effectively adding yet another abstraction for time,
> > which is something I'd hope to avoid unless you have a strong reason other
> > than it being faster in theory.
>
> Excellent idea. It turns out to be precisely 0 (see below). A
> motivation still remains, though: this allows comparison with units
> specified in terms of jiffies, which means that the unit being
> compared matches the exact tick of the clock, making those comparisons
> as precise as possible, for what they are. I suppose you could argue,
> on the other hand, that nanoseconds give so much precision already,
> that approximations using them amount practically to the same thing.
> I'm not sure which way to reason about that.
>
> For interest, here are a few comparisons taken with kbench9000:
>
> get_jiffies_boot_64 26
> ktime_get_coarse_boottime 26
> ktime_get_boot_fast_ns with tsc 70
> ktime_get_boot_fast_ns with hpet 4922
> ktime_get_boot_fast_ns with acpi_pm 1884
>
> As expected, hpet is really quite painful.

I would prefer not to add the new interface then. We might in
fact move users of get_jiffies_64() to ktime_get_coarse() for
consistency given the small overhead of that function.

      Arnd