linux-kernel - Re: oprofile and ARM A9 hardware counter

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Date:	Wed, 15 Feb 2012 17:38:20 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	eranian@...il.com, "Shilimkar, Santosh" <santosh.shilimkar@...com>,
	David Long <dave.long@...aro.org>, b-cousson@...com,
	mans@...sr.com, will.deacon@....com,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: oprofile and ARM A9 hardware counter

On Wed, 2012-02-08 at 10:24 +0800, Ming Lei wrote:
> 
> On OMAP4, HZ is 128, and perf_rotate_context may set a new sample period(~8ms),
> which is much longer than 1ms in 1000HZ freq mode, so less sample events are
> observed. X86 isn't affected since its HZ is 1000.
> 
> With patch[1], about 10000 sample events can be generated after running
> 'perf record -e cycles  ./noploop 10' and 'perf report -D | tail -20'
> on panda board.


> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 32b48c8..db4faf2 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2300,14 +2300,12 @@ do {                                    \
>         return div64_u64(dividend, divisor);
>  }
> 
> -static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
> +static void perf_adjust_period(struct perf_event *event, u64 period, u64 count)
>  {
>         struct hw_perf_event *hwc = &event->hw;
> -       s64 period, sample_period;
> +       s64 sample_period;
>         s64 delta;
> 
> -       period = perf_calculate_period(event, nsec, count);
> -
>         delta = (s64)(period - hwc->sample_period);
>         delta = (delta + 7) / 8; /* low pass filter */
> 
> @@ -2363,8 +2361,13 @@ static void perf_ctx_adjust_freq(struct
> perf_event_context *ctx, u64 period)
>                 delta = now - hwc->freq_count_stamp;
>                 hwc->freq_count_stamp = now;
> 
> -               if (delta > 0)
> +               if (delta > 0) {
> +                       period = perf_calculate_period(event, period, delta);
> +
> +                       if (period > 4*hwc->sample_period)
> +                               period = hwc->sample_period;
>                         perf_adjust_period(event, period, delta);
> +               }
>         }
>  }
> 
> @@ -4533,8 +4536,10 @@ static int __perf_event_overflow(struct
> perf_event *event,
> 
>                 hwc->freq_time_stamp = now;
> 
> -               if (delta > 0 && delta < 2*TICK_NSEC)
> +               if (delta > 0 && delta < 2*TICK_NSEC) {
> +                       delta = perf_calculate_period(event, delta, hwc->last_period);
>                         perf_adjust_period(event, delta, hwc->last_period);
> +               }
>         }
> 
>         /* 

So what this patch seems to do is put that filter on period in
perf_ctx_adjust_freq(). Not making sense.. nor can I see a HZ
dependency, perf_ctx_adjust_freq() uses TICK_NSEC as time base.
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