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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 25, 2019 at 05:08:33PM +0300, Alexander Shishkin wrote:
> +static void perf_aux_sample_output(struct perf_event *event,
> +				   struct perf_output_handle *handle,
> +				   struct perf_sample_data *data)
> +{
> +	struct perf_event *sampler = event->aux_event;
> +	unsigned long pad;
> +	struct ring_buffer *rb;
> +	long size;
> +
> +	if (WARN_ON_ONCE(!sampler || !data->aux_size))
> +		return;
> +
> +	rb = ring_buffer_get(sampler->parent ? sampler->parent : sampler);
> +	if (!rb)
> +		return;
> +
> +	/*
> +	 * Guard against NMI hits inside the critical section;
> +	 * see also perf_aux_sample_size().
> +	 */
> +	WRITE_ONCE(rb->aux_in_sampling, 1);
> +
> +	size = perf_pmu_aux_sample_output(sampler, handle, data->aux_size);
> +
> +	/*
> +	 * An error here means that perf_output_copy() failed (returned a
> +	 * non-zero surplus that it didn't copy), which in its current
> +	 * enlightened implementation is not possible. If that changes, we'd
> +	 * like to know.
> +	 */
> +	if (WARN_ON_ONCE(size < 0))
> +		goto out_clear;
> +
> +	/*
> +	 * The pad comes from ALIGN()ing data->aux_size up to u64 in
> +	 * perf_aux_sample_size(), so should not be more than that.
> +	 */
> +	pad = data->aux_size - size;
> +	if (WARN_ON_ONCE(pad >= sizeof(u64)))
> +		pad = 8;
> +
> +	if (pad) {
> +		u64 zero = 0;
> +		perf_output_copy(handle, &zero, pad);
> +	}
> +
> +out_clear:
> +	WRITE_ONCE(rb->aux_in_sampling, 0);
> +
> +	ring_buffer_put(rb);
> +}

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() ?

---
--- 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)
 {
 	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();
 
-	size = perf_pmu_aux_sample_output(sampler, handle, data->aux_size);
+	size = perf_pmu_snapshot_aux(sampler, handle, data->aux_size);
 
 	/*
 	 * An error here means that perf_output_copy() failed (returned a
@@ -6335,7 +6336,7 @@ static void perf_aux_sample_output(struc
 
 	/*
 	 * The pad comes from ALIGN()ing data->aux_size up to u64 in
-	 * perf_aux_sample_size(), so should not be more than that.
+	 * perf_prepare_sample_aux(), so should not be more than that.
 	 */
 	pad = data->aux_size - size;
 	if (WARN_ON_ONCE(pad >= sizeof(u64)))
@@ -6347,6 +6348,7 @@ static void perf_aux_sample_output(struc
 	}
 
 out_clear:
+	barrier();
 	WRITE_ONCE(rb->aux_in_sampling, 0);
 
 	ring_buffer_put(rb);
@@ -6881,7 +6883,7 @@ void perf_prepare_sample(struct perf_eve
 		size = min_t(size_t, U16_MAX - header->size,
 			     event->attr.aux_sample_size);
 		size = rounddown(size, 8);
-		size = perf_aux_sample_size(event, data, size);
+		size = perf_prepare_sample_aux(event, data, size);
 
 		WARN_ON_ONCE(size + header->size > U16_MAX);
 		header->size += size;