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

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Message-ID: <CAK8P3a3DpRvk1Mw_MKs8wAbRJbMUQoY2UTgK1CF8UOiBQg=btw@mail.gmail.com>
Date:   Thu, 13 Jun 2019 17:40:15 +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 Thu, Jun 13, 2019 at 5:19 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Hey Arnd, Peter,
>
> On Wed, Jun 12, 2019 at 4:01 PM Arnd Bergmann <arnd@...db.de> wrote:
> > 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.
>
> Oh, thanks. Indeed ktime_get_coarse_boottime seems even better. It's
> perhaps a bit slower, in that it has that seqlock, but that might give
> better synchronization between CPUs as well.

A seqlock is a very cheap synchronization primitive, I would actually
guess that this is faster than most implementations of sched_clock()
that access a hardware register for reading the time.

       Arnd