From: Arnd Bergmann <arnd@arndb.de>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
Clemens Ladisch <clemens@ladisch.de>,
Sultan Alsawaf <sultan@kerneltoast.com>,
Waiman Long <longman@redhat.com>, X86 ML <x86@kernel.org>
Subject: Re: infinite loop in read_hpet from ktime_get_boot_fast_ns
Date: Wed, 12 Jun 2019 16:01:04 +0200 [thread overview]
Message-ID: <CAK8P3a15NTV=njOjz-ccYL8=_q_MdEru0A+jeE=f7ufUTOOTgw@mail.gmail.com> (raw)
In-Reply-To: <CAHmME9obwzZ5x=p3twDfNYux+kg0h4QAGe0ePAkZ2KqvguBK3g@mail.gmail.com>
On Wed, Jun 12, 2019 at 11:46 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> On Wed, Jun 12, 2019 at 11:03 AM Peter Zijlstra <peterz@infradead.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
next prev parent reply other threads:[~2019-06-12 14:01 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-07 14:14 infinite loop in read_hpet from ktime_get_boot_fast_ns Jason A. Donenfeld
2019-06-11 21:09 ` Thomas Gleixner
2019-06-11 21:40 ` Waiman Long
2019-06-12 9:02 ` Peter Zijlstra
2019-06-12 9:44 ` Jason A. Donenfeld
2019-06-12 12:28 ` Peter Zijlstra
2019-06-12 12:31 ` Peter Zijlstra
2019-06-12 12:58 ` Jason A. Donenfeld
2019-06-12 15:27 ` Peter Zijlstra
2019-06-12 19:46 ` Arnd Bergmann
2019-06-18 17:34 ` Jason A. Donenfeld
2019-06-12 14:01 ` Arnd Bergmann [this message]
2019-06-13 15:18 ` Jason A. Donenfeld
2019-06-13 15:40 ` Arnd Bergmann
2019-06-13 16:17 ` Jason A. Donenfeld
2019-06-13 16:26 ` Thomas Gleixner
2019-06-13 16:34 ` Jason A. Donenfeld
2019-06-13 16:41 ` Jason A. Donenfeld
2019-06-13 19:53 ` Thomas Gleixner
2019-06-14 9:14 ` Jason A. Donenfeld
2019-06-14 9:44 ` Thomas Gleixner
2019-06-14 9:56 ` Jason A. Donenfeld
2019-06-14 9:48 ` [PATCH] timekeeping: add get_jiffies_boot_64() for jiffies including sleep Jason A. Donenfeld
2019-06-14 9:55 ` [tip:timers/urgent] timekeeping: Repair ktime_get_coarse*() granularity tip-bot for Thomas Gleixner
2019-06-14 11:18 ` Arnd Bergmann
2019-06-12 9:29 ` infinite loop in read_hpet from ktime_get_boot_fast_ns Peter Zijlstra
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