Messages in this thread | | | Date | Mon, 26 Apr 2010 15:18:43 +0200 | From | Johannes Stezenbach <> | Subject | Re: Q: sched_clock() vs. clocksource, how to implement correctly |
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On Sat, Apr 24, 2010 at 10:50:11PM +0200, Johannes Stezenbach wrote: > /* > * Orion's sched_clock implementation. It has a resolution of > * at least 7.5ns (133MHz TCLK) and a maximum value of 834 days. > * > * Because the hardware timer period is quite short (21 secs if > * 200MHz TCLK) and because cnt32_to_63() needs to be called at > * least once per half period to work properly, a kernel timer is > * set up to ensure this requirement is always met. > */ > #define TCLK2NS_SCALE_FACTOR 8
I found the following discussion of the sched_clock() implementation trade-offs very informative: http://lkml.org/lkml/2010/4/15/299
It mentions clocksource_calc_mult_shift() which was added in 2.6.33, however I had some difficulties understanding the meaning of the minsec parameter, especially since all existing callers use a value of 4. But when using minsec = 365*24*60*60 (1 year) it results in the shift value of 8.
So finally the pieces connect together :-)
> BTW, even though this uses TCLK2NS_SCALE_FACTOR of 8, the same file > uses a shift vaue of 20 for the orion_clksrc...
It seems clocksource_cyc2ns() is used in kernel/time/timekeeping.c and kernel/time/clocksource.c only on relatively small delta values, so there's no need to worry about overflow and a large clocksource.shift and .mult is OK.
(Apparently the minsec value of 4 mentioned above is suitable for timekeeping? Where does the 4 come from?)
Thanks, Johannes
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