linux-kernel - Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with

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Date:	Wed, 6 Jan 2016 19:12:22 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Prarit Bhargava <prarit@...hat.com>
cc:	John Stultz <john.stultz@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Xunlei Pang <pang.xunlei@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Baolin Wang <baolin.wang@...aro.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/2] kernel, timekeeping, add trylock option to
 ktime_get_with_offset()

On Wed, 6 Jan 2016, Prarit Bhargava wrote:
> On 01/06/2016 12:34 PM, Thomas Gleixner wrote:
> > On Wed, 6 Jan 2016, John Stultz wrote:
> >> On Wed, Jan 6, 2016 at 5:00 AM, Prarit Bhargava <prarit@...hat.com> wrote:
> >>> -ktime_t ktime_get_with_offset(enum tk_offsets offs)
> >>> +ktime_t ktime_get_with_offset(enum tk_offsets offs, int trylock)
> >>>  {
> >>>         struct timekeeper *tk = &tk_core.timekeeper;
> >>>         unsigned int seq;
> >>>         ktime_t base, *offset = offsets[offs];
> >>>         s64 nsecs;
> >>> +       unsigned long flags = 0;
> >>> +
> >>> +       if (unlikely(!timekeeping_initialized))
> >>> +               return ktime_set(0, 0);
> >>>
> >>>         WARN_ON(timekeeping_suspended);
> >>>
> >>> +       if (trylock && !raw_spin_trylock_irqsave(&timekeeper_lock, flags))
> >>> +               return ktime_set(KTIME_MAX, 0);
> >>
> >> Wait.. this doesn't make sense. The timekeeper lock is only for reading.
> >>
> >> What I was suggesting to you off line is to have something that avoids
> >> spinning on the seqcounter should if a bug occurs and we IPI all the
> >> cpus, that we don't deadlock or block any printk messages.
> > 
> > We could also extend the fast timekeeper with boot/real/tai extensions and use
> > that for printk. You can use ktime_get_mono_fast_ns() today.
> > 
> 
> Thanks tglx -- I thought about doing that but was put off by the comments
> in __ktime_get_fast_ns() which point out that we could see backwards time
> stamps.  But I see your point -- I could do the same "last_time_stamp" check
> and use "??" in the output.

We talk about single digit nanoseconds here and in the case of a crash/bug we
really do not care about that at all.

Thanks,

	tglx
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