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

[Leo Yan]

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@xxxxxxxxxx>
> ---
>  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
>