linux-kernel - Re: [PATCH v4 1/7] perf: Allow normal events to be sources of AUX data

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Message-ID: <20190805131856.GM2349@hirez.programming.kicks-ass.net>
Date:   Mon, 5 Aug 2019 15:18:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        kan.liang@...ux.intel.com, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v4 1/7] perf: Allow normal events to be sources of AUX
 data

On Mon, Aug 05, 2019 at 10:16:12AM +0300, Alexander Shishkin wrote:
> In some cases, ordinary (non-AUX) events can generate data for AUX events.
> For example, PEBS events can come out as records in the Intel PT stream
> instead of their usual DS records, if configured to do so.
> 
> One requirement for such events is to consistently schedule together, to
> ensure that the data from the "AUX source" events isn't lost while their
> corresponding AUX event is not scheduled. We use grouping to provide this
> guarantee: an "AUX source" event can be added to a group where an AUX event
> is a group leader, and provided that the former supports writing to the
> latter.

Two niggles -- and sorry for not bringing those up sooner:

> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 7198ddd0c6b1..213cae95e713 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -374,7 +374,8 @@ struct perf_event_attr {
>  				namespaces     :  1, /* include namespaces data */
>  				ksymbol        :  1, /* include ksymbol events */
>  				bpf_event      :  1, /* include bpf events */
> -				__reserved_1   : 33;
> +				aux_source     :  1, /* generate AUX records instead of events */
> +				__reserved_1   : 32;
>  
>  	union {
>  		__u32		wakeup_events;	  /* wakeup every n events */

The name: "aux_source" seems to imply that we change the source to
'aux', while in fact it is the destination we change.

So we want to %s/aux_source/aux_output/ on the whole thing?

> +	/*
> +	 * Our group leader must be an aux event if we want to be
> +	 * an aux_source. This way, the aux event will precede its
> +	 * aux_source events in the group, and therefore will always
> +	  schedule first.
> +	 */

You fudged the comment there.