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

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Date:   Mon, 28 Oct 2019 19:10:21 +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:

> On Mon, Oct 28, 2019 at 05:27:12PM +0100, Peter Zijlstra wrote:
>> 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() ?
>
> That is, given the previous delta, the below.
>
> ---
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6292,9 +6292,17 @@ long perf_pmu_snapshot_aux(struct perf_e
>  	 * IRQs need to be disabled to prevent IPIs from racing with us.
>  	 */
>  	local_irq_save(flags);
> +	/*
> +	 * Guard against NMI hits inside the critical section;
> +	 * see also perf_prepare_sample_aux().
> +	 */
> +	WRITE_ONCE(rb->aux_in_sampling, 1);
> +	barrier();
>  
>  	ret = event->pmu->snapshot_aux(event, handle, size);
>  
> +	barrier();
> +	WRITE_ONCE(rb->aux_in_sampling, 0);
>  	local_irq_restore(flags);
>  
>  	return ret;
> @@ -6316,13 +6324,6 @@ static void perf_aux_sample_output(struc
>  	if (!rb)
>  		return;
>  
> -	/*
> -	 * Guard against NMI hits inside the critical section;
> -	 * see also perf_prepare_sample_aux().
> -	 */
> -	WRITE_ONCE(rb->aux_in_sampling, 1);
> -	barrier();
> -
>  	size = perf_pmu_snapshot_aux(sampler, handle, data->aux_size);
>  
>  	/*
> @@ -6348,9 +6349,6 @@ static void perf_aux_sample_output(struc
>  	}
>  
>  out_clear:
> -	barrier();
> -	WRITE_ONCE(rb->aux_in_sampling, 0);
> -
>  	ring_buffer_put(rb);

I can't tell without applying these, if the labels still make sense. But
this one probably becomes "out_put" at this point.

Thanks,
--
Alex