Re: [RFC PATCH 1/1] kernel/events: Introduce IOC_COUNT_RECORDS

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Jun 06, 2017 at 08:46:28PM +0530, Naveen N. Rao escreveu:
> Many perf sideband events (context switches, namespaces, ...) are useful
> by themselves without the need for subscribing to any overflow events.
> However, it is not possible to subscribe for notifications when such
> records are logged into the ring buffer. Introduce IOC_COUNT_RECORDS as
> a way to request this.
> 
> With IOC_COUNT_RECORDS set, IOC_REFRESH takes the number of records
> after which to generate a notification, rather than the number of
> overflow events.

Can you take a look at tools/perf/python/twatch.py?

[acme@jouet linux]$ make O=/tmp/build/perf -C tools/perf install-bin
[root@jouet linux]# export PYTHONPATH=/tmp/build/perf/python/
[root@jouet linux]# python tools/perf/python/twatch.py
cpu:  0, pid: 29860, tid: 29860 { type: exit, pid: 29860, ppid: 29860, tid: 29860, ptid: 29860, time: 117729363047027}
cpu:  0, pid: 29854, tid: 29854 { type: exit, pid: 29854, ppid: 29854, tid: 29854, ptid: 29854, time: 117729363617885}
cpu:  0, pid: 29853, tid: 29853 { type: fork, pid: 29865, ppid: 29853, tid: 29865, ptid: 29853, time: 117729363800225}
cpu:  0, pid: 29865, tid: 29865 { type: comm, pid: 29865, tid: 29865, comm: fixdep }
cpu:  0, pid: 29865, tid: 29865 { type: exit, pid: 29865, ppid: 29865, tid: 29865, ptid: 29865, time: 117729364898505}
cpu:  0, pid: 29853, tid: 29853 { type: fork, pid: 29866, ppid: 29853, tid: 29866, ptid: 29853, time: 117729365022416}
cpu:  0, pid: 29866, tid: 29866 { type: comm, pid: 29866, tid: 29866, comm: rm }
cpu:  0, pid: 29866, tid: 29866 { type: exit, pid: 29866, ppid: 29866, tid: 29866, ptid: 29866, time: 117729365665831}
cpu:  0, pid: 29853, tid: 29853 { type: fork, pid: 29867, ppid: 29853, tid: 29867, ptid: 29853, time: 117729365846030}
cpu:  0, pid: 29867, tid: 29867 { type: comm, pid: 29867, tid: 29867, comm: mv }
cpu:  2, pid: 28218, tid: 28218 { type: exit, pid: 28218, ppid: 28218, tid: 28218, ptid: 28218, time: 117729704900029}
^CTraceback (most recent call last):
  File "tools/perf/python/twatch.py", line 68, in <module>
    main()
  File "tools/perf/python/twatch.py", line 40, in main
    evlist.poll(timeout = -1)
KeyboardInterrupt
[root@jouet linux]#

This is using the python binding to get notifications for such meta
events "synchronously", you can do the same with a C proggie, of course,
and using just what we have in the kernel already.

See its changelog comments to see examples:

git log tools/perf/python/twatch.py

For instance, what I think you want is in:

[acme@jouet linux]$ git log --oneline -1 cfeb1d90a1b1db96383b48888cb7a5f10ca12e12
cfeb1d90a1b1 perf python: Use attr.watermark in twatch.py
[acme@jouet linux]$ git log --oneline -1 58b32c1b538f2d197ce385d6a314e83f8b787021
58b32c1b538f perf python: Make twatch.py use soft dummy event, freq=0
[acme@jouet linux]$

- Arnaldo
 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
>  include/linux/perf_event.h      |  1 +
>  include/uapi/linux/perf_event.h |  1 +
>  kernel/events/core.c            | 16 +++++++++++++++-
>  kernel/events/ring_buffer.c     |  9 +++++++++
>  4 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 24a635887f28..016f2da2bba7 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -683,6 +683,7 @@ struct perf_event {
>  	struct irq_work			pending;
>  
>  	atomic_t			event_limit;
> +	bool				count_records;
>  
>  	/* address range filters */
>  	struct perf_addr_filters_head	addr_filters;
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index b1c0b187acfe..fb989ac71ded 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -408,6 +408,7 @@ struct perf_event_attr {
>  #define PERF_EVENT_IOC_ID		_IOR('$', 7, __u64 *)
>  #define PERF_EVENT_IOC_SET_BPF		_IOW('$', 8, __u32)
>  #define PERF_EVENT_IOC_PAUSE_OUTPUT	_IOW('$', 9, __u32)
> +#define PERF_EVENT_IOC_COUNT_RECORDS	_IO ('$', 10)
>  
>  enum perf_event_ioc_flags {
>  	PERF_IOC_FLAG_GROUP		= 1U << 0,
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 6e75a5c9412d..637064880b36 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2674,6 +2674,16 @@ void perf_event_addr_filters_sync(struct perf_event *event)
>  }
>  EXPORT_SYMBOL_GPL(perf_event_addr_filters_sync);
>  
> +static int _perf_event_count_records(struct perf_event *event)
> +{
> +	if (event->attr.inherit || !is_sampling_event(event))
> +		return -EINVAL;
> +
> +	event->count_records = 1;
> +
> +	return 0;
> +}
> +
>  static int _perf_event_refresh(struct perf_event *event, int refresh)
>  {
>  	/*
> @@ -4699,6 +4709,9 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
>  		func = _perf_event_reset;
>  		break;
>  
> +	case PERF_EVENT_IOC_COUNT_RECORDS:
> +		return _perf_event_count_records(event);
> +
>  	case PERF_EVENT_IOC_REFRESH:
>  		return _perf_event_refresh(event, arg);
>  
> @@ -7342,7 +7355,8 @@ static int __perf_event_overflow(struct perf_event *event,
>  	 */
>  
>  	event->pending_kill = POLL_IN;
> -	if (events && atomic_dec_and_test(&event->event_limit)) {
> +	if (events && !event->count_records &&
> +			atomic_dec_and_test(&event->event_limit)) {
>  		ret = 1;
>  		event->pending_kill = POLL_HUP;
>  
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 2831480c63a2..9b9ca0608fed 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -126,6 +126,7 @@ __perf_output_begin(struct perf_output_handle *handle,
>  		u64			 id;
>  		u64			 lost;
>  	} lost_event;
> +	int events = atomic_read(&event->event_limit);
>  
>  	rcu_read_lock();
>  	/*
> @@ -197,6 +198,14 @@ __perf_output_begin(struct perf_output_handle *handle,
>  	if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
>  		local_add(rb->watermark, &rb->wakeup);
>  
> +	if (events && event->count_records &&
> +			atomic_dec_and_test(&event->event_limit)) {
> +		event->pending_kill = POLL_HUP;
> +		local_inc(&rb->wakeup);
> +
> +		perf_event_disable_inatomic(event);
> +	}
> +
>  	page_shift = PAGE_SHIFT + page_order(rb);
>  
>  	handle->page = (offset >> page_shift) & (rb->nr_pages - 1);
> -- 
> 2.12.2