Re: [PATCH v2] perf/core: Add a new read format to get a number of lost samples

Jiri Olsa
2021-09-10T21:42:38Z

On Wed, Sep 08, 2021 at 10:24:20AM -0700, Namhyung Kim wrote:
> Sometimes we want to know an accurate number of samples even if it's
> lost. Currenlty PERF_RECORD_LOST is generated for a ring-buffer which
> might be shared with other events. So it's hard to know per-event
> lost count.
>
> Add event->lost_samples field and PERF_FORMAT_LOST to retrieve it from
> userspace.

hi,
looks good.. will there be some tools/perf change using this?

thanks,
jirka

>
> Original-patch-by: Jiri Olsa <jolsa@redhat.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
> include/linux/perf_event.h | 2 ++
> include/uapi/linux/perf_event.h | 5 ++++-
> kernel/events/core.c | 22 +++++++++++++++++++---
> kernel/events/ring_buffer.c | 5 ++++-
> 4 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index f5a6a2f069ed..189a471fba42 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -756,6 +756,8 @@ struct perf_event {
> struct pid_namespace *ns;
> u64 id;
>
> + atomic64_t lost_samples;
> +
> u64 (*clock)(void);
> perf_overflow_handler_t overflow_handler;
> void *overflow_handler_context;
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index bf8143505c49..f72008949ff0 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -299,6 +299,7 @@ enum {
> * { u64 time_enabled; } && PERF_FORMAT_TOTAL_TIME_ENABLED
> * { u64 time_running; } && PERF_FORMAT_TOTAL_TIME_RUNNING
> * { u64 id; } && PERF_FORMAT_ID
> + * { u64 lost; } && PERF_FORMAT_LOST
> * } && !PERF_FORMAT_GROUP
> *
> * { u64 nr;
> @@ -306,6 +307,7 @@ enum {
> * { u64 time_running; } && PERF_FORMAT_TOTAL_TIME_RUNNING
> * { u64 value;
> * { u64 id; } && PERF_FORMAT_ID
> + * { u64 lost; } && PERF_FORMAT_LOST
> * } cntr[nr];
> * } && PERF_FORMAT_GROUP
> * };
> @@ -315,8 +317,9 @@ enum perf_event_read_format {
> PERF_FORMAT_TOTAL_TIME_RUNNING = 1U << 1,
> PERF_FORMAT_ID = 1U << 2,
> PERF_FORMAT_GROUP = 1U << 3,
> + PERF_FORMAT_LOST = 1U << 4,
>
> - PERF_FORMAT_MAX = 1U << 4, /* non-ABI */
> + PERF_FORMAT_MAX = 1U << 5, /* non-ABI */
> };
>
> #define PERF_ATTR_SIZE_VER0 64 /* sizeof first published struct */
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 0e125ae2fa92..8708325ee4a2 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1841,6 +1841,9 @@ static void __perf_event_read_size(struct perf_event *event, int nr_siblings)
> if (event->attr.read_format & PERF_FORMAT_ID)
> entry += sizeof(u64);
>
> + if (event->attr.read_format & PERF_FORMAT_LOST)
> + entry += sizeof(u64);
> +
> if (event->attr.read_format & PERF_FORMAT_GROUP) {
> nr += nr_siblings;
> size += sizeof(u64);
> @@ -5255,11 +5258,15 @@ static int __perf_read_group_add(struct perf_event *leader,
> values[n++] += perf_event_count(leader);
> if (read_format & PERF_FORMAT_ID)
> values[n++] = primary_event_id(leader);
> + if (read_format & PERF_FORMAT_ID)
> + values[n++] = atomic64_read(&leader->lost_samples);
>
> for_each_sibling_event(sub, leader) {
> values[n++] += perf_event_count(sub);
> if (read_format & PERF_FORMAT_ID)
> values[n++] = primary_event_id(sub);
> + if (read_format & PERF_FORMAT_ID)
> + values[n++] = atomic64_read(&sub->lost_samples);
> }
>
> raw_spin_unlock_irqrestore(&ctx->lock, flags);
> @@ -5316,7 +5323,7 @@ static int perf_read_one(struct perf_event *event,
> u64 read_format, char __user *buf)
> {
> u64 enabled, running;
> - u64 values[4];
> + u64 values[5];
> int n = 0;
>
> values[n++] = __perf_event_read_value(event, &enabled, &running);
> @@ -5326,6 +5333,8 @@ static int perf_read_one(struct perf_event *event,
> values[n++] = running;
> if (read_format & PERF_FORMAT_ID)
> values[n++] = primary_event_id(event);
> + if (read_format & PERF_FORMAT_LOST)
> + values[n++] = atomic64_read(&event->lost_samples);
>
> if (copy_to_user(buf, values, n * sizeof(u64)))
> return -EFAULT;
> @@ -5664,6 +5673,7 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
>
> return perf_event_modify_attr(event, &new_attr);
> }
> +
> default:
> return -ENOTTY;
> }
> @@ -6835,7 +6845,7 @@ static void perf_output_read_one(struct perf_output_handle *handle,
> u64 enabled, u64 running)
> {
> u64 read_format = event->attr.read_format;
> - u64 values[4];
> + u64 values[5];
> int n = 0;
>
> values[n++] = perf_event_count(event);
> @@ -6849,6 +6859,8 @@ static void perf_output_read_one(struct perf_output_handle *handle,
> }
> if (read_format & PERF_FORMAT_ID)
> values[n++] = primary_event_id(event);
> + if (read_format & PERF_FORMAT_LOST)
> + values[n++] = atomic64_read(&event->lost_samples);
>
> __output_copy(handle, values, n * sizeof(u64));
> }
> @@ -6859,7 +6871,7 @@ static void perf_output_read_group(struct perf_output_handle *handle,
> {
> struct perf_event *leader = event->group_leader, *sub;
> u64 read_format = event->attr.read_format;
> - u64 values[5];
> + u64 values[6];
> int n = 0;
>
> values[n++] = 1 + leader->nr_siblings;
> @@ -6877,6 +6889,8 @@ static void perf_output_read_group(struct perf_output_handle *handle,
> values[n++] = perf_event_count(leader);
> if (read_format & PERF_FORMAT_ID)
> values[n++] = primary_event_id(leader);
> + if (read_format & PERF_FORMAT_LOST)
> + values[n++] = atomic64_read(&leader->lost_samples);
>
> __output_copy(handle, values, n * sizeof(u64));
>
> @@ -6890,6 +6904,8 @@ static void perf_output_read_group(struct perf_output_handle *handle,
> values[n++] = perf_event_count(sub);
> if (read_format & PERF_FORMAT_ID)
> values[n++] = primary_event_id(sub);
> + if (read_format & PERF_FORMAT_LOST)
> + values[n++] = atomic64_read(&sub->lost_samples);
>
> __output_copy(handle, values, n * sizeof(u64));
> }
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 52868716ec35..727ca8f4caad 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -172,8 +172,10 @@ __perf_output_begin(struct perf_output_handle *handle,
> goto out;
>
> if (unlikely(rb->paused)) {
> - if (rb->nr_pages)
> + if (rb->nr_pages) {
> local_inc(&rb->lost);
> + atomic64_inc(&event->lost_samples);
> + }
> goto out;
> }
>
> @@ -254,6 +256,7 @@ __perf_output_begin(struct perf_output_handle *handle,
>
> fail:
> local_inc(&rb->lost);
> + atomic64_inc(&event->lost_samples);
> perf_output_put_handle(handle);
> out:
> rcu_read_unlock();
> --
> 2.33.0.309.g3052b89438-goog
>