Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with_offset()

[Petr Mladek]

On Wed 2016-01-06 08:00:33, Prarit Bhargava wrote:
> This is a timekeeping staging patch for the printk() timestamp
> functionality that adds a trylock option for the timekeeping_lock() to
> ktime_get_with_offset().  When trylock is 1, calls to
> ktime_get_with_offset() will return return a ktime of 0 if the
> timekeeping_lock is locked.

If I get it correctly, it returns 0 when timekeeping is not
initialized. But it returns TIME_MAX_NS when the lock is taken.
Where TIME_MAX_NS is defined in the 2nd patch.

> This patch adds ktime_try_real(), ktime_try_boot(), and ktime_try_tai() as
> wrapper functions around ktime_get_with_offset() with trylock = 1, and
> modifies other callers to call ktime_get_with_offset() with trylock = 0.
> 
> ---
>  include/linux/timekeeping.h |   50 +++++++++++++++++++++++++++++++++++++++----
>  kernel/time/timekeeping.c   |   15 ++++++++++++-
>  2 files changed, 60 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
> index ec89d84..4f47352 100644
> --- a/include/linux/timekeeping.h
> +++ b/include/linux/timekeeping.h
> @@ -166,7 +166,7 @@ enum tk_offsets {
>  };
>  
>  extern ktime_t ktime_get(void);
> -extern ktime_t ktime_get_with_offset(enum tk_offsets offs);
> +extern ktime_t ktime_get_with_offset(enum tk_offsets offs, int trylock);
>  extern ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs);
>  extern ktime_t ktime_get_raw(void);
>  extern u32 ktime_get_resolution_ns(void);
> @@ -176,7 +176,16 @@ extern u32 ktime_get_resolution_ns(void);
>   */
>  static inline ktime_t ktime_get_real(void)
>  {
> -	return ktime_get_with_offset(TK_OFFS_REAL);
> +	return ktime_get_with_offset(TK_OFFS_REAL, 0);
> +}
> +
> +/**
> + * ktime_try_real - same as ktime_get_real, except return 0 if timekeeping is
> + * locked.
> + */
> +static inline ktime_t ktime_try_real(void)

I would call this ktime_try_get_real() to make it clear.


> +{
> +	return ktime_get_with_offset(TK_OFFS_REAL, 1);
>  }
>  
[...]

>  static inline u64 ktime_get_raw_ns(void)
>  {
>  	return ktime_to_ns(ktime_get_raw());
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index d563c19..6e2cbeb 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -44,6 +44,8 @@ static struct {
>  static DEFINE_RAW_SPINLOCK(timekeeper_lock);
>  static struct timekeeper shadow_timekeeper;
>  
> +/* printk may call ktime_get_with_offset() before timekeeping is initialized. */
> +static int timekeeping_initialized;
>  /**
>   * struct tk_fast - NMI safe timekeeper
>   * @seq:	Sequence counter for protecting updates. The lowest bit
> @@ -705,15 +707,22 @@ static ktime_t *offsets[TK_OFFS_MAX] = {
>  	[TK_OFFS_TAI]	= &tk_core.timekeeper.offs_tai,
>  };
>  
> -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);
> +

I guess that you want to avoid a deadlock with this. I mean that you
want to survive when you call, for example, ktime_try_tai_ns() from
inside timekeeping_set_tai_offset(). Am I right?

One problem is that it will fail even when the lock is taken from
another CPU and the deadlock is not real. It probably is not a big
issue for printk() because currently used local_clock() is far from perfect
but...

Another problem is that it will block writers. This might be solved
if you try only one while cycle instead of taking the lock.
I mean to do something like:

ktime_t ktime_get_with_offset(enum tk_offsets offs, int try_once)
{
	struct timekeeper *tk = &tk_core.timekeeper;
	unsigned int seq;
	int retry;
	ktime_t base, *offset = offsets[offs];
	s64 nsecs;

	WARN_ON(timekeeping_suspended);

	do {
		seq = read_seqcount_begin(&tk_core.seq);
		base = ktime_add(tk->tkr_mono.base, *offset);
		nsecs = timekeeping_get_ns(&tk->tkr_mono);
		retry = read_seqcount_retry(&tk_core.seq, seq));
	} while (retry && !try_once);

	if (try_once && retry)
		return ktime_set(KTIME_MAX, 0);

	return ktime_add_ns(base, nsecs);

}

Another question is if you really need to distinguish between
non-initialized and locked state. You might always return zero
time if you do not know. It will things easier.


>  	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));
>  
> +	if (trylock)
> +		raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
> +
>  	return ktime_add_ns(base, nsecs);
>  

Best Regards,
Petr
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