Messages in this thread | | | Date | Wed, 1 Apr 2020 09:24:17 +0800 | From | Leo Yan <> | Subject | Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer |
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On Fri, Mar 20, 2020 at 05:35:45PM +0800, Leo Yan wrote: > Between the system powering on and kernel's sched clock registration, > the arch timer usually has been enabled at the early time and its > counter is incremented during the period of the booting up. Thus the > arch timer's counter is not completely accounted into the sched clock, > and has a delta between the arch timer's counter and sched clock. This > delta value should be stored into userpg->time_offset, which later can > be retrieved by Perf tool in the user space for sample timestamp > calculation. > > Now userpg->time_offset is assigned to the negative sched clock with > '-now', this value cannot reflect the delta between arch timer's counter > and sched clock, so Perf cannot use it to calculate the sample time. > > To fix this issue, this patch calculate the delta between the arch > timer's and sched clock and assign the delta to userpg->time_offset. > The detailed steps are firstly to convert counter to nanoseconds 'ns', > then the offset is calculated as 'now' minus 'ns'. > > |<------------------- 'ns' ---------------------->| > |<-------- 'now' -------->| > |<---- time_offset ---->| > |-----------------------|-------------------------| > ^ ^ ^ > Power on system sched clock registration Perf starts > > Signed-off-by: Leo Yan <leo.yan@linaro.org> > --- > arch/arm64/kernel/perf_event.c | 19 ++++++++++++++++++- > 1 file changed, 18 insertions(+), 1 deletion(-)
Gentle ping ...
Hi Mike R., Peter,
If possible, could you give a look for this patch?
Thank you, Leo
> > diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c > index e40b65645c86..226d25d77072 100644 > --- a/arch/arm64/kernel/perf_event.c > +++ b/arch/arm64/kernel/perf_event.c > @@ -1143,6 +1143,7 @@ void arch_perf_update_userpage(struct perf_event *event, > { > u32 freq; > u32 shift; > + u64 count, ns, quot, rem; > > /* > * Internal timekeeping for enabled/running/stopped times > @@ -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; > } > -- > 2.17.1 >
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