Re: [PATCH] clocksource: document some basic concepts

[Peter Zijlstra]

On Mon, 2010-11-15 at 11:33 +0100, Linus Walleij wrote:
> +sched_clock()
> +-------------
> +
> +In addition to the clock sources and clock events there is a special weak
> +function in the kernel called sched_clock(). This function shall return the
> +number of nanoseconds since the system was started. An architecture may or
> +may not provide an implementation of sched_clock() on its own.
> +
> +As the name suggests, sched_clock() is used for scheduling the system,
> +determining the absolute timeslice for a certain process in the CFS scheduler
> +for example. It is also used for printk timestamps when you have selected to
> +include time information in printk for things like bootcharts.
> +
> +Compared to clock sources, sched_clock() has to be very fast: it is called
> +much more often, especially by the scheduler. If you have to do trade-offs
> +between accuracy compared to the clock source, you may sacrifice accuracy
> +for speed in sched_clock(). It however require the same basic characteristics
> +as the clock source, i.e. it has to be monotonic.

Not so, we prefer it be synchronized and monotonic, but we don't require
so, see below.

> +The sched_clock() function may wrap only on unsigned long long boundaries,
> +i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps
> +after circa 585 years. (For most practical systems this means "never".)

Currently true, John Stultz was going to look into ammending this by
teaching the kernel/sched_clock.c bits about early wraps (and a way for
architectures to specify this)

#define SCHED_CLOCK_WRAP_BITS 48

...

#ifdef SCHED_CLOCK_WRAP_BITS
  /* handle short wraps */
#endif

foo for wrap_min/wrap_max and "delta = now - scd->tick_raw" like things
might work.

> +If an architecture does not provide its own implementation of this function,
> +it will fall back to using jiffies, making its maximum resolution 1/HZ of the
> +jiffy frequency for the architecture. This will affect scheduling accuracy
> +and will likely show up in system benchmarks. 

sched_clock() need not be synchronized between CPUs, nor even be
monotonic, we prefer a fast high res clock over a slow one,
CONFIG_HAVE_UNSTABLE_SCHED_CLOCK provides infrastructure to sanitize the
output of sched_clock().

[ of course we prefer a fast and synchronized clock, but we take fast
over synchronized ]

sched_clock() requires local IRQs to be disabled.

Therefore, sched_clock() shall not be used, see kernel/sched_clock.c for
detail and alternative interfaces.
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