linux-kernel - Re: [PATCH 2/3] perf events: Introduce realtime clock event

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Date:	Fri, 18 Feb 2011 12:14:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Ahern <daahern@...co.com>
Cc:	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu, acme@...stprotocols.net, paulus@...ba.org
Subject: Re: [PATCH 2/3] perf events: Introduce realtime clock event

On Thu, 2011-02-17 at 22:53 -0700, David Ahern wrote:
> The motivation for this event is to convert perf_clock() time stamps
> to wall-clock (gettimeofday()) equivalents, including adjustments made
> by NTP (e.g., for comparing perf events to other log files).

> This patch is based on the monotonic patch by Arnaldo Carvalho de Melo
> <acme@...hat.com>.
> 
> (NOTE: Comments from the last review of the timehist patch series
> suggested calling this a monotonic clock. I am not trying to be
> dense here; since gettimeofday maps to realtime clock I think that
> is the better name for it.)

Well, the idea was to use CLOCK_MONOTONIC, not to call CLOCK_REALTIME
monotonic.

I'm really not sure why you want CLOCK_REALTIME and I think
CLOCK_MONOTONIC is more useful (I'd argue you want your system logs to
contain both, every admin who's ever had to untangle what happened
during DST switches will agree)

> @@ -5610,6 +5612,13 @@ static enum hrtimer_restart perf_swevent_hrtimer(struct hrtimer *hrtimer)
>  
>  	perf_sample_data_init(&data, 0);
>  	data.period = event->hw.last_period;
> +	if (event->attr.sample_type & PERF_SAMPLE_RAW) 
> +	{
> +		raw.size = sizeof(u64);
> +		raw.data = &event->count;
> +		data.raw = &raw;
> +	}
> +
>  	regs = get_irq_regs();
>  
>  	if (regs && !perf_exclude_event(event, regs)) {


Why!? you already keep ->count = ktime_get_real(), so simply reading the
count value will get you the timestamp.. this is superfluous at best.


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