Re: [PATCH 5/7] perf: Document aux api usage

[Mathieu Poirier]

On 3 December 2015 at 03:32, Alexander Shishkin
<alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
> In order to ensure safe aux buffer management, we rely on the assumption
> that pmu::stop() stops its ongoing aux transaction and not just the hw.
>
> This patch documents this requirement for perf_aux_output_{begin,end}()
> apis.
>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> ---
>  kernel/events/ring_buffer.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 6865ac95ca..1aed2617e8 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -252,6 +252,10 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
>   * The ordering is similar to that of perf_output_{begin,end}, with
>   * the exception of (B), which should be taken care of by the pmu
>   * driver, since ordering rules will differ depending on hardware.
> + *
> + * Call this from pmu::start(); see the comment in perf_aux_output_end()
> + * about its use in pmu callbacks. Both can also be called from the PMI
> + * handler if needed.
>   */
>  void *perf_aux_output_begin(struct perf_output_handle *handle,
>                             struct perf_event *event)
> @@ -323,6 +327,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
>         return handle->rb->aux_priv;
>
>  err_put:
> +       /* can't be last */
>         rb_free_aux(rb);
>
>  err:
> @@ -337,6 +342,10 @@ err:
>   * aux_head and posting a PERF_RECORD_AUX into the perf buffer. It is the
>   * pmu driver's responsibility to observe ordering rules of the hardware,
>   * so that all the data is externally visible before this is called.
> + *
> + * Note: this has to be called from pmu::stop() callback, as the assumption
> + * of the aux buffer management code is that after pmu::stop(), the aux
> + * transaction must be stopped and therefore drop the aux reference count.
>   */
>  void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size,
>                          bool truncated)
> @@ -376,6 +385,7 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size,
>         handle->event = NULL;
>
>         local_set(&rb->aux_nest, 0);
> +       /* can't be last */
>         rb_free_aux(rb);
>         ring_buffer_put(rb);
>  }
> --
> 2.6.2
>

Thanks for the heads-up.  My next version (V7) will follow the same scheme.

Mathieu
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