From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
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 14:58:21 +0200 [thread overview]
Message-ID: <CAHmME9oWhWi=Gp2RpM0AOO+_1_24znUxDkz6CyJTc2qRgRRivw@mail.gmail.com> (raw)
In-Reply-To: <20190612122843.GJ3436@hirez.programming.kicks-ass.net>
Hi Peter,
Thanks for the explanation.
On Wed, Jun 12, 2019 at 2:29 PM Peter Zijlstra <peterz@infradead.org> wrote:
> Either local_clock() or cpu_clock(cpu). The sleep hooks are not
> something the consumer has to worry about.
Alright. Just so long as it *is* tracking sleep, then that's fine. If
it isn't some important aspects of the protocol will be violated.
> If an architecture doesn't provide a sched_clock(), you're on a
> seriously handicapped arch. It wraps in ~500 days, and aside from
> changing jiffies_lock to a latch, I don't think we can do much about it.
Are you sure? The base definition I'm looking at uses jiffies:
unsigned long long __weak sched_clock(void)
{
return (unsigned long long)(jiffies - INITIAL_JIFFIES)
* (NSEC_PER_SEC / HZ);
}
On a CONFIG_HZ_1000 machine, jiffies wraps in ~49.7 days:
>>> ((1<<32)-1)/1000/(60*60*24)
49.710269618055555
Why not just use get_jiffies_64()? The lock is too costly on 32bit?
> (the scheduler too expects sched_clock() to not wrap short of the u64
> and so having those machines online for 500 days will get you 'funny'
> results)
Ahh. So if, on the other hand, the whole machine explodes at the wrap
mark, I guess my silly protocol is the least of concerns, and so this
shouldn't matter?
Jason
next prev parent reply other threads:[~2019-06-12 12:58 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 [this message]
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
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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