Re: [PATCH v2 0/3] arm64: perf_event: Fix time offset prior to epoch

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 11, 2020 at 11:22:00AM +0200, Peter Zijlstra wrote:

> (_completely_ untested)
> 
> ---
>  arch/arm64/kernel/perf_event.c | 27 ++++++++++++++++++---------
>  include/linux/sched_clock.h    | 28 ++++++++++++++++++++++++++++
>  kernel/time/sched_clock.c      | 41 +++++++++++++----------------------------
>  3 files changed, 59 insertions(+), 37 deletions(-)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index 4d7879484cec..81a49a916660 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -1165,28 +1165,37 @@ device_initcall(armv8_pmu_driver_init)
>  void arch_perf_update_userpage(struct perf_event *event,
>  			       struct perf_event_mmap_page *userpg, u64 now)
>  {
> -	u32 freq;
> -	u32 shift;
> +	struct clock_read_data *rd;
> +	unsigned int seq;
>  
>  	/*
>  	 * Internal timekeeping for enabled/running/stopped times
>  	 * is always computed with the sched_clock.
>  	 */
> -	freq = arch_timer_get_rate();
>  	userpg->cap_user_time = 1;
> +	userpg->cap_user_time_zero = 1;
> +
> +	do {
> +		rd = sched_clock_read_begin(&seq);
> +
> +		userpg->time_mult = rd->mult;
> +		userpg->time_shift = rd->shift;
> +		userpg->time_offset = rd->epoch_ns;

			^^^^^^^ wants to be time_zero

> +
> +		userpg->time_zero -= (rd->epoch_cyc * rd->shift) >> rd->shift;
> +
> +	} while (sched_clock_read_retry(seq));
> +
> +	userpg->time_offset = userpf->time_zero - now;
>  
> -	clocks_calc_mult_shift(&userpg->time_mult, &shift, freq,
> -			NSEC_PER_SEC, 0);

And that ^^^ was complete crap.

>  	/*
>  	 * time_shift is not expected to be greater than 31 due to
>  	 * the original published conversion algorithm shifting a
>  	 * 32-bit value (now specifies a 64-bit value) - refer
>  	 * perf_event_mmap_page documentation in perf_event.h.
>  	 */
> -	if (shift == 32) {
> -		shift = 31;
> +	if (userpg->time_shift == 32) {
> +		userpg->time_shift = 31;
>  		userpg->time_mult >>= 1;
>  	}
> -	userpg->time_shift = (u16)shift;
> -	userpg->time_offset = -now;
>  }