On Thu, Apr 30, 2020 at 06:27:50PM +0200, Peter Zijlstra wrote: > On Thu, Apr 30, 2020 at 03:58:24PM +0100, Will Deacon wrote: > > On Fri, Mar 20, 2020 at 05:35:45PM +0800, Leo Yan wrote: > > > + /* > > > + * Since arch timer is enabled ealier than sched clock registration, > > > + * compuate the delta (in nanosecond unit) between the arch timer > > > + * counter and sched clock, assign the delta to time_offset and > > > + * perf tool can use it for timestamp calculation. > > > + * > > > + * The formula for conversion arch timer cycle to ns is: > > > + * quot = (cyc >> time_shift); > > > + * rem = cyc & ((1 << time_shift) - 1); > > > + * ns = quot * time_mult + ((rem * time_mult) >> time_shift); > > > + */ > > > + count = arch_timer_read_counter(); > > > + quot = count >> shift; > > > + rem = count & ((1 << shift) - 1); > > > + ns = quot * userpg->time_mult + ((rem * userpg->time_mult) >> shift); > > > + userpg->time_offset = now - ns; > > > > Hmm, reading the counter and calculating the delta feels horribly > > approximate to me. It would be much better if we could get hold of the > > initial epoch cycles from the point at which sched_clock was initialised > > using the counter. This represents the true cycle delta between the counter > > and what sched_clock uses for 0 ns. > > > > Unfortunately, I can't see a straightforward way to grab that information. > > It looks like x86 pulls this directly from the TSC driver. > > Yeah, and I'm thinking you should do the same. IIRC ARM uses this > kernel/time/sched_clock.c thing, and if I read that right, the struct > clock_data there has all the bits you need here. > > So I'm thinking that you might want to add a helper function here to get > you the good stuff.
Thanks, Peter. Leo -- do you think you could look at implementing this as part of a v2, please? Will
