Messages in this thread | | | Date | Wed, 8 Dec 2010 15:05:17 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM |
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On Wed, Dec 08, 2010 at 03:44:19PM +0100, Peter Zijlstra wrote: > One of the problems is I think the cycles2ns multiplication of the raw > clock, that makes dealing with wrap-around lots harder, so I guess we > should deal with the wrap on the raw clock values and then apply > cycles2ns on the delta or somesuch. But I expect the clocksource > infrastructure already has something like that, John?
I've thought about that, but it becomes slightly problematical, as was shown in one of the examples I provided. If you do scale by doing a 64-bit multiply and shift, you're always going to end up with less than a 64-bit result.
I think your idea makes sense though, but I think for it to be able to cover the full 64-bit range, we need to do the wraparound handling after scaling. So maybe something like the following:
static unsigned long long last_ns, cur_ns; static unsigned long long max = (max_read_clock() * mult) >> shift;
unsigned long long sched_clock(void) { unsigned long long cyc = read_clock(); unsigned long long ns = (cyc * mult) >> shift; unsigned long long delta;
spin_lock(&sched_clock_lock); delta = last_ns - ns; if (ns < last_ns) delta += max;
last_ns = ns; ns = cur_ns += delta; spin_unlock(&sched_clock_lock);
return ns; }
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