Make timehands count selectable at boottime?

[Sebastian Huber]

On 01/10/2019 16:37, Konstantin Belousov wrote:
> On Tue, Oct 01, 2019 at 12:48:46PM +0200, Sebastian Huber wrote:
>> Hello,
>>
>> a recent commit made the timehands count selectable at boottime:
>>
>> https://reviews.freebsd.org/D21563
>>
>> What was the reason for this change?
> 
> See the followup thread on the svn-src@ list for this revision.

Thanks for the hint. So, this change is related to the PPS handling and 
the things which happen between a pps_capture() and pps_event().

What I don't understand is why more timehands are a problem for the 
consumer side. For example:

void
binuptime(struct bintime *bt)
{
	struct timehands *th;
	u_int gen;

	do {
		th = timehands;
		gen = atomic_load_acq_int(&th->th_generation);
		*bt = th->th_offset;
		bintime_addx(bt, th->th_scale * tc_delta(th));
		atomic_thread_fence_acq();
	} while (gen == 0 || gen != th->th_generation);
}

With more timehands the time frame in which th->th_generation remains 
constant is longer the more timehands you have. So, it is less likely 
that you have to execute the loop body twice. The count of timehands is 
limited by the timecounter frequency and mask, and the hz configuration.

-- 
Sebastian Huber, embedded brains GmbH

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