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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: Thu, 13 Jun 2019 17:40:15 +0200	[thread overview]
Message-ID: <CAK8P3a3DpRvk1Mw_MKs8wAbRJbMUQoY2UTgK1CF8UOiBQg=btw@mail.gmail.com> (raw)
In-Reply-To: <CAHmME9pOWk_ZteUZc_PT19rMn1kfYcXtmLcyAy5sncdV1tNuiQ@mail.gmail.com>

On Thu, Jun 13, 2019 at 5:19 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hey Arnd, Peter,
>
> On Wed, Jun 12, 2019 at 4:01 PM Arnd Bergmann <arnd@arndb.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

  reply	other threads:[~2019-06-13 15:40 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
2019-06-13 15:18         ` Jason A. Donenfeld
2019-06-13 15:40           ` Arnd Bergmann [this message]
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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