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

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Message-ID: <20190612090257.GF3436@hirez.programming.kicks-ass.net>
Date:   Wed, 12 Jun 2019 11:02:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        LKML <linux-kernel@...r.kernel.org>, clemens@...isch.de,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Waiman Long <longman@...hat.com>, x86@...nel.org
Subject: Re: infinite loop in read_hpet from ktime_get_boot_fast_ns

On Tue, Jun 11, 2019 at 11:09:20PM +0200, Thomas Gleixner wrote:
> Jason,
> 
> On Fri, 7 Jun 2019, Jason A. Donenfeld wrote:
> 
> Adding a few more people on cc and keeping full context.
> 
> > Hey Thomas,
> > 
> > After some discussions here prior about the different clocks
> > available, WireGuard uses ktime_get_boot_fast_ns() pretty extensively.
> > The requirement is for a quasi-accurate monotonic counter that takes
> > into account sleep time, and this seems to fit the bill pretty well.

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.