linux-kernel - Re: infinite loop in read_hpet from ktime_get_boot_fast

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Date:   Wed, 12 Jun 2019 16:01:04 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Clemens Ladisch <clemens@...isch.de>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Waiman Long <longman@...hat.com>, X86 ML <x86@...nel.org>
Subject: Re: infinite loop in read_hpet from ktime_get_boot_fast_ns

On Wed, Jun 12, 2019 at 11:46 AM Jason A. Donenfeld <Jason@...c4.com> wrote:
> On Wed, Jun 12, 2019 at 11:03 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > How quasi? Do the comments in kernel/sched/clock.c look like something
> > you could use?
> >
> > As already mentioned in the other tasks, anything ktime will be
> > horrifically crap when it ends up using the HPET, the code in
> > kernel/sched/clock.c is a best effort to keep using TSC even when it is
> > deemed unusable for timekeeping.
>
> Thanks for pointing that out. Indeed the HPET path is a bummer and I'd
> like to just escape using ktime all together.
>
> In fact, my accuracy requirements are very lax. I could probably even
> deal with an inaccuracy as huge as ~200 milliseconds. But what I do
> need is 64-bit, so that it doesn't wrap, allowing me to compare two
> stamps taken a long time apart, and for it to take into account sleep
> time, like CLOCK_BOOTTIME does, which means get_jiffies_64() doesn't
> fit the bill. I was under the impression that I could only get this
> with ktime_get_boot & co, because those add the sleep offset.

Documentation/core-api/timekeeping.rst describes the timekeeping
interfaces. I think what you want here is ktime_get_coarse_boottime().

Note that "coarse" means "don't access the hardware clocksource"
here, which is faster than "fast", but less accurate.

This is updated as often as "jiffies_64", but is in nanosecond resolution
and takes suspended time into account.

      Arnd