linux-kernel - Re: [PATCH v3 1/3] perf: Allow using AUX data in perf samples

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Date:   Mon, 28 Oct 2019 19:08:18 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        jolsa@...hat.com, adrian.hunter@...el.com,
        mathieu.poirier@...aro.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH v3 1/3] perf: Allow using AUX data in perf samples

Peter Zijlstra <peterz@...radead.org> writes:

> I have the below delta on top of this patch.
>
> And while I get why we need recursion protection for pmu::snapshot_aux,
> I'm a little puzzled on why it is over the padding, that is, why isn't
> the whole of aux_in_sampling inside (the newly minted)
> perf_pmu_snapshot_aux() ?

No reason. Too long staring at that code by myself.

> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6237,7 +6237,7 @@ perf_output_sample_ustack(struct perf_ou
>  	}
>  }
>  
> -static unsigned long perf_aux_sample_size(struct perf_event *event,
> +static unsigned long perf_prepare_sample_aux(struct perf_event *event,
>  					  struct perf_sample_data *data,
>  					  size_t size)
>  {
> @@ -6275,9 +6275,9 @@ static unsigned long perf_aux_sample_siz
>  	return data->aux_size;
>  }
>  
> -long perf_pmu_aux_sample_output(struct perf_event *event,
> -				struct perf_output_handle *handle,
> -				unsigned long size)
> +long perf_pmu_snapshot_aux(struct perf_event *event,
> +			   struct perf_output_handle *handle,
> +			   unsigned long size)

That makes more sense indeed.

>  {
>  	unsigned long flags;
>  	long ret;
> @@ -6318,11 +6318,12 @@ static void perf_aux_sample_output(struc
>  
>  	/*
>  	 * Guard against NMI hits inside the critical section;
> -	 * see also perf_aux_sample_size().
> +	 * see also perf_prepare_sample_aux().
>  	 */
>  	WRITE_ONCE(rb->aux_in_sampling, 1);
> +	barrier();

Isn't WRITE_ONCE() barrier enough on its own? My thinking was that we
only need a compiler barrier here, hence the WRITE_ONCE.

Thanks,
--
Alex