Messages in this thread | | | Subject | Re: [PATCH 04/10] tile: convert to use clocksource_register_hz | From | john stultz <> | Date | Thu, 11 Nov 2010 14:17:15 -0800 |
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On Thu, 2010-11-11 at 16:21 -0500, Chris Metcalf wrote: > On 11/1/2010 4:12 PM, John Stultz wrote: > > Convert tile to use clocksource_register_hz. > > > > Untested. Help from maintainers would be appreciated. > > > > CC: Chris Metcalf <cmetcalf@tilera.com> > > CC: Thomas Gleixner <tglx@linutronix.de> > > Signed-off-by: John Stultz <johnstul@us.ibm.com> > > --- > > arch/tile/kernel/time.c | 5 +---- > > 1 files changed, 1 insertions(+), 4 deletions(-) > > > > diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c > > index 6bed820..8c06cb2 100644 > > --- a/arch/tile/kernel/time.c > > +++ b/arch/tile/kernel/time.c > > @@ -76,7 +76,6 @@ static struct clocksource cycle_counter_cs = { > > .rating = 300, > > .read = clocksource_get_cycles, > > .mask = CLOCKSOURCE_MASK(64), > > - .shift = 22, /* typical value, e.g. x86 tsc uses this */ > > .flags = CLOCK_SOURCE_IS_CONTINUOUS, > > }; > > We were using clocksource_calc_mult_shift() for a while to compute both > this value and our sched_clock() value, and, like > clocksource_register_hz(), it suggested that a shift value of 31 was best. > > Perhaps unsurprisingly, we saw wraparound with sched_clock(), which is why > we use a fixed shift value of "10" to avoid problems. This is clearly > required since sched_clock() values are fairly long-lived.
One more quick thing: I just reviewed the tile sched_clock code, and it looks like its doing the right thing, calculating a mult/shift pair for the sched_clock code itself, not directly using the timekeeping clocksource's values.
So the clocksource_register_hz patch proposed here doesn't really affect this issue and should still be fine to go in (assuming no other issues are brought out by it :).
But Chris' point does still illustrate that we need better clarification about what is expected from sched_clock and maybe some better reference implementations to make sure arch maintainers don't trip on common issues when implementing or modifying sched_clock.
thanks -john
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