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

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Message-ID: <CAK8P3a2oLUKjY+3Qki59ruygzSb1Vsaoo5Et3BecGzpG8-=tOA@mail.gmail.com>
Date:   Wed, 19 Jun 2019 22:57:11 +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 10:07 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> On Wed, Jun 19, 2019 at 10:02 PM Arnd Bergmann <arnd@...db.de> wrote:
> > > 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.
>
> In light of the measurements, that seems like a good plan to me.
>
> One thing to consider with moving jiffies users over that way is
> ktime_t. Do you want to introduce helpers like
> ktime_get_boot_coarse_ns(), just like there is already with the other
> various functions like ktime_get_boot_ns(), ktime_get_boot_fast_ns(),
> etc? (I'd personally prefer using the _ns variants, at least.) I can
> send a patch for this.

That sounds reasonable, but then I think we should have the full
set of coarse_*_ns() functions, again for consistency:

                u64 ktime_get_coarse_ns(void)
                u64 ktime_get_coarse_boottime_ns(void)
                u64 ktime_get_coarse_real_ns(void)
                u64 ktime_get_coarse_clocktai_ns(void)

and document them in Documentation/core-api/timekeeping.rst.

We seem to also be lacking the basic ktime_get_coarse(), which
seems like a major omission.
Both ktime_get_coarse_ns and ktime_get_coarse can be wrappers
around ktime_get_coarse_ts64() then, while the others would
use ktime_get_coarse_with_offset().

       Arnd