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

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Date:	Wed, 6 Jan 2016 09:28:09 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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, 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.


> +
>         do {
>                 seq = read_seqcount_begin(&tk_core.seq);
>                 base = ktime_add(tk->tkr_mono.base, *offset);
> @@ -721,6 +730,9 @@ ktime_t ktime_get_with_offset(enum tk_offsets offs)
>
>         } while (read_seqcount_retry(&tk_core.seq, seq));

So instead of the do/while() loop above... Something closer to:

int __ktime_get_with_offset(enum tk_offsets offs, ktime_t* base, s64 *nsec)
{
       unsigned int seq;

       seq = read_seqcount_begin(&tk_core.seq);
       *base = ktime_add(tk->tkr_mono.base, *offset);
       *nsecs = timekeeping_get_ns(&tk->tkr_mono);

       return read_seqcount_retry(&tk_core.seq, seq);
}

Then ktime_get_with_offset() would just call:
      ...
      while(__ktime_get_with_offset(offs, &base, &nsecs))
                          /*spin*/;
      return ktime_add_ns(base,nsecs);

Then you add a simple:
int ktime_try_get_with_offset(enum tk_offsets offs, ktime_t* ret)
{
   ...
    if (__ktime_get_with_offset(offs, &base, &nsecs))
           return -EAGAIN;
    *ret = ktime_add_ns(base,nsecs);
     return 0;
}


thanks
-john
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