Messages in this thread | | | Subject | Re: [PATCH] clocksource: document some basic concepts | From | Peter Zijlstra <> | Date | Mon, 15 Nov 2010 11:48:14 +0100 |
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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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