Messages in this thread | | | Date | Thu, 28 May 2009 17:44:44 +0900 | From | Paul Mundt <> | Subject | Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock(). |
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On Wed, May 27, 2009 at 09:25:25AM -0700, Daniel Walker wrote: > On Wed, 2009-05-27 at 09:15 +0900, Paul Mundt wrote: > > > unsigned long long __attribute__((weak)) sched_clock(void) > > { > > + /* > > + * Use the current clocksource when it becomes available later in > > + * the boot process, and ensure that it is usable for sched_clock(). > > + */ > > + if (clock && (clock->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK)) > > + return cyc2ns(clock, clocksource_read(clock)); > > + > > + /* Otherwise just fall back on jiffies */ > > return (unsigned long long)(jiffies - INITIAL_JIFFIES) > > Do we really need all this complexity in a fast path? the jiffies > clocksource is static and always ready. Could we instead remove the > usage of "clock" and create a new pointer called "sched_clocksource" and > initialize it to the jiffies clock, and allow the clocksource management > code to update that new pointer based on the > CLOCK_SOURCE_USE_FOR_SCHED_CLOCK flag when new clocksources are > registered/removed/marked unstable etc.. > > that would eliminate all the code in sched_clock except one line, > > unsigned long long __attribute__((weak)) sched_clock(void) > { > return cyc2ns(sched_clocksource, clocksource_read(sched_clocksource)); > } > It would, but then what would that do to the sched_clock() value? It will reset once a CLOCK_SOURCE_USE_FOR_SCHED_CLOCK clocksource comes along, which basically means that sched_clock() then rolls back, which is worse than simply being delayed a bit. So even if we were to use a special sched_clocksource, it still could not be used until it was set up at registration time.
Now we could of course add a sched_clocksource assignment in select_clocksource(), but that's really no different than just testing 'clock' as we do today. All it does is move the flag test in to a different path.
So, I think the current v3 is still the best solution.
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