Messages in this thread | | | Date | Thu, 30 Apr 2020 18:27:50 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer |
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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:
> > @@ -1164,5 +1165,21 @@ void arch_perf_update_userpage(struct perf_event *event, > > userpg->time_mult >>= 1; > > } > > userpg->time_shift = (u16)shift; > > - userpg->time_offset = -now; > > + > > + /* > > + * 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.
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