Subject: Re: [PATCH 2/2] powerpc/perf/hv-24x7: Display change in counter values

From: Madhavan Srinivasan <[email-masked]>
Date: Tue, 2 Feb 2016 23:47:33 +0530

On Saturday 30 January 2016 08:37 AM, Sukadev Bhattiprolu wrote:
> From a1aa992fb25fb8e98a5c5724376ae8cc91463de3 Mon Sep 17 00:00:00 2001
> From: Sukadev Bhattiprolu <[email-masked]>
> Date: Mon, 25 Jan 2016 23:05:36 -0500
> Subject: [PATCH 2/2] powerpc/perf/hv-24x7: Display change in counter values
>
> For 24x7 counters, perf displays the raw value of the 24x7 counter, which
> is a monotonically increasing value.
>
> 	perf stat -C 0 -e \
> 		'hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/' \
> 		sleep 1
>
>  Performance counter stats for 'CPU(s) 0':
>
>      9,105,403,170      hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/
>
>        0.000425751 seconds time elapsed
>
> In the typical usage of 'perf stat' this counter value is not as useful
> as the _change_ in the counter value over the duration of the application.

This may break application using this interface right? i.e, since
for all this time, counter output was raw values and application
may be post processing to calculate the difference, now with
this patch, application may need some change? Also,
should not this be documented somewhere?

Maddy

> Have h_24x7_event_init() set the event's prev_count to the raw value of
> the 24x7 counter at the time of initialization. When the application
> terminates, hv_24x7_event_read() will compute the change in value and
> report to the perf tool. Similarly, for the transaction interface, clear
> the event count to 0 at the beginning of the transaction.
>
> 	perf stat -C 0 -e \
> 		'hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/' \
> 		sleep 1
>
>  Performance counter stats for 'CPU(s) 0':
>
>            245,758      hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/
>
>        1.006366383 seconds time elapsed
>
> Signed-off-by: Sukadev Bhattiprolu <[email-masked]>
> ---
>  arch/powerpc/perf/hv-24x7.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/perf/hv-24x7.c b/arch/powerpc/perf/hv-24x7.c
> index b7a9a03..77b958f 100644
> --- a/arch/powerpc/perf/hv-24x7.c
> +++ b/arch/powerpc/perf/hv-24x7.c
> @@ -1222,11 +1222,12 @@ static int h_24x7_event_init(struct perf_event *event)
>  		return -EACCES;
>  	}
>  
> -	/* see if the event complains */
> +	/* Get the initial value of the counter for this event */
>  	if (single_24x7_request(event, &ct)) {
>  		pr_devel("test hcall failed\n");
>  		return -EIO;
>  	}
> +	(void)local64_xchg(&event->hw.prev_count, ct);
>  
>  	return 0;
>  }
> @@ -1289,6 +1290,16 @@ static void h_24x7_event_read(struct perf_event *event)
>  			h24x7hw = &get_cpu_var(hv_24x7_hw);
>  			h24x7hw->events[i] = event;
>  			put_cpu_var(h24x7hw);
> +			/*
> +			 * Clear the event count so we can compute the _change_
> +			 * in the 24x7 raw counter value at the end of the txn.
> +			 *
> +			 * Note that we could alternatively read the 24x7 value
> +			 * now and save its value in event->hw.prev_count. But
> +			 * that would require issuing a hcall, which would then
> +			 * defeat the purpose of using the txn interface.
> +			 */
> +			local64_set(&event->count, 0);
>  		}
>  
>  		put_cpu_var(hv_24x7_reqb);