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SubjectRe: [PATCH] arm64: perf_event: Fix time_offset for arch timer
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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