From: Peter Zijlstra <peterz@infradead.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
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 17:27:30 +0200 [thread overview]
Message-ID: <20190612152730.GI3402@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHmME9oWhWi=Gp2RpM0AOO+_1_24znUxDkz6CyJTc2qRgRRivw@mail.gmail.com>
On Wed, Jun 12, 2019 at 02:58:21PM +0200, Jason A. Donenfeld wrote:
> 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.
The scheduler also cares about how long a task has been sleeping, so
yes, that's automagic.
> > 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
Bah, I must've done the math wrong (or assumed HZ=100).
> Why not just use get_jiffies_64()? The lock is too costly on 32bit?
Deadlocks when you do get_jiffies_64() from within an update. What would
be an easier update is forcing everyone to use the GENERIC_SCHED_CLOCK
fallback or something like that.
OTOH, changing jiffies_lock to a latch shouldn't be rocket science
either.
> > (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?
That was my thinking...
next prev parent reply other threads:[~2019-06-12 15:36 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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190612152730.GI3402@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=Jason@zx2c4.com \
--cc=clemens@ladisch.de \
--cc=linux-kernel@vger.kernel.org \
--cc=longman@redhat.com \
--cc=sultan@kerneltoast.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).