Re: [RFC] perf/core: Add an ioctl to get a number of lost samples

From: Jiri Olsa
Date: Wed Aug 11 2021 - 11:04:49 EST


On Tue, Aug 10, 2021 at 11:21:35PM -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_EVENT_IOC_LOST_SAMPLES to
> retrieve it from userspace.
>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> include/linux/perf_event.h | 2 ++
> include/uapi/linux/perf_event.h | 1 +
> kernel/events/core.c | 9 +++++++++
> kernel/events/ring_buffer.c | 5 ++++-
> 4 files changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index f5a6a2f069ed..44d72079c77a 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;
>
> + atomic_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..24397799127d 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -505,6 +505,7 @@ struct perf_event_query_bpf {
> #define PERF_EVENT_IOC_PAUSE_OUTPUT _IOW('$', 9, __u32)
> #define PERF_EVENT_IOC_QUERY_BPF _IOWR('$', 10, struct perf_event_query_bpf *)
> #define PERF_EVENT_IOC_MODIFY_ATTRIBUTES _IOW('$', 11, struct perf_event_attr *)
> +#define PERF_EVENT_IOC_LOST_SAMPLES _IOR('$', 12, __u64 *)

would it be better to use the read syscall for that?
https://lore.kernel.org/lkml/20210622153918.688500-5-jolsa@xxxxxxxxxx/

that patchset ended up on me not having a way to reproduce the
issue you guys wanted the fix for ;-) the lost count is there
as well

jirka

>
> enum perf_event_ioc_flags {
> PERF_IOC_FLAG_GROUP = 1U << 0,
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 0e125ae2fa92..a4d6736b6594 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5664,6 +5664,15 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
>
> return perf_event_modify_attr(event, &new_attr);
> }
> +
> + case PERF_EVENT_IOC_LOST_SAMPLES: {
> + u64 lost = atomic_read(&event->lost_samples);
> +
> + if (copy_to_user((void __user *)arg, &lost, sizeof(lost)))
> + return -EFAULT;
> + return 0;
> + }
> +
> default:
> return -ENOTTY;
> }
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 52868716ec35..06d7dacb05da 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);
> + atomic_inc(&event->lost_samples);
> + }
> goto out;
> }
>
> @@ -254,6 +256,7 @@ __perf_output_begin(struct perf_output_handle *handle,
>
> fail:
> local_inc(&rb->lost);
> + atomic_inc(&event->lost_samples);
> perf_output_put_handle(handle);
> out:
> rcu_read_unlock();
> --
> 2.32.0.605.g8dce9f2422-goog
>