linux-kernel - Re: [PATCH v1 1/4] perf: Allow suppressing AUX records

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Date:   Wed, 15 Nov 2017 13:00:22 +0100
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,
        Will Deacon <will.deacon@....com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Markus Metzger <markus.t.metzger@...el.com>
Subject: Re: [PATCH v1 1/4] perf: Allow suppressing AUX records

On Tue, Nov 14, 2017 at 02:30:21PM +0200, Alexander Shishkin wrote:
> It has been pointed out to me many times that it is useful to be able
> to switch off AUX records to save the bandwidth for records that actually
> matter, for example, in AUX overwrite mode.
> 
> The usefulness of PERF_RECORD_AUX is in some of its flags, like the
> TRUNCATED flag that tells the decoder where exactly gaps in the trace are.
> The OVERWRITE flag, on the other hand will be set on every single record
> in overwrite mode. However, a PERF_RECORD_AUX[flags=OVERWRITE] is
> generated on every target task's sched_out, which over time adds up to
> a lot of useless information.
> 
> This patch adds an attribute bit that enables suppressing such records.

What this fails to explain is why a separate suppression flag makes
sense. Why not suppress the thing on overwrite mode and be done with it?

> +		if (!handle->event->attr.suppress_aux ||
> +		    (handle->aux_flags & ~(u64)SUPPRESSABLE_FLAGS))
> +			perf_event_aux_event(handle->event, aux_head, size,
> +					     handle->aux_flags);

That wants { }