Messages in this thread | | | From | Arnd Bergmann <> | Date | Wed, 19 Jun 2019 22:02:07 +0200 | Subject | Re: [PATCH v2] timekeeping: get_jiffies_boot_64() for jiffies that include sleep time |
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On Wed, Jun 19, 2019 at 5:31 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > Hi Arnd, > > On Wed, Jun 19, 2019 at 5:08 PM Arnd Bergmann <arnd@arndb.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
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