Re: [PATCH 10/46] perf core: Introduce new ioctl options to pause and resume ring buffer

[Arnaldo Carvalho de Melo]

Em Fri, Feb 26, 2016 at 09:31:58AM +0000, Wang Nan escreveu:
> Add new ioctl() to pause/resume ring-buffer output.
> 
> In some situations we want to read from ring buffer only when we
> ensure nothing can write to the ring buffer during reading. Without
> this patch we have to turn off all events attached to this ring buffer
> to achieve this.
> 
> This patch is for supporting overwrite ring buffer. Following
> commits will introduce new methods support reading from overwrite ring
> buffer. Before reading caller must ensure the ring buffer is frozen, or
> the reading is unreliable.

Peter, have you have the chance too look at this and the other kernel
bits in this kit?

- Arnaldo
 
> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> Cc: He Kuang <hekuang@xxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Brendan Gregg <brendan.d.gregg@xxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Zefan Li <lizefan@xxxxxxxxxx>
> Cc: pi3orama@xxxxxxx
> ---
>  include/uapi/linux/perf_event.h |  1 +
>  kernel/events/core.c            | 13 +++++++++++++
>  kernel/events/internal.h        | 11 +++++++++++
>  kernel/events/ring_buffer.c     |  7 ++++++-
>  4 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 1afe962..a3c1903 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -401,6 +401,7 @@ struct perf_event_attr {
>  #define PERF_EVENT_IOC_SET_FILTER	_IOW('$', 6, char *)
>  #define PERF_EVENT_IOC_ID		_IOR('$', 7, __u64 *)
>  #define PERF_EVENT_IOC_SET_BPF		_IOW('$', 8, __u32)
> +#define PERF_EVENT_IOC_PAUSE_OUTPUT	_IOW('$', 9, __u32)
>  
>  enum perf_event_ioc_flags {
>  	PERF_IOC_FLAG_GROUP		= 1U << 0,
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 94c47e3..a7075ae 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4231,6 +4231,19 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
>  	case PERF_EVENT_IOC_SET_BPF:
>  		return perf_event_set_bpf_prog(event, arg);
>  
> +	case PERF_EVENT_IOC_PAUSE_OUTPUT: {
> +		struct ring_buffer *rb;
> +
> +		rcu_read_lock();
> +		rb = rcu_dereference(event->rb);
> +		if (!event->rb) {
> +			rcu_read_unlock();
> +			return -EINVAL;
> +		}
> +		rb_toggle_paused(rb, !!arg);
> +		rcu_read_unlock();
> +		return 0;
> +	}
>  	default:
>  		return -ENOTTY;
>  	}
> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index 2bbad9c..6a93d1b 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -18,6 +18,7 @@ struct ring_buffer {
>  #endif
>  	int				nr_pages;	/* nr of data pages  */
>  	int				overwrite;	/* can overwrite itself */
> +	int				paused;		/* can write into ring buffer */
>  
>  	atomic_t			poll;		/* POLL_ for wakeups */
>  
> @@ -65,6 +66,16 @@ static inline void rb_free_rcu(struct rcu_head *rcu_head)
>  	rb_free(rb);
>  }
>  
> +static inline void
> +rb_toggle_paused(struct ring_buffer *rb,
> +		 bool pause)
> +{
> +	if (!pause && rb->nr_pages)
> +		rb->paused = 0;
> +	else
> +		rb->paused = 1;
> +}
> +
>  extern struct ring_buffer *
>  rb_alloc(int nr_pages, long watermark, int cpu, int flags);
>  extern void perf_event_wakeup(struct perf_event *event);
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 1faad2c..22e1a47 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -125,8 +125,11 @@ int perf_output_begin(struct perf_output_handle *handle,
>  	if (unlikely(!rb))
>  		goto out;
>  
> -	if (unlikely(!rb->nr_pages))
> +	if (unlikely(rb->paused)) {
> +		if (rb->nr_pages)
> +			local_inc(&rb->lost);
>  		goto out;
> +	}
>  
>  	handle->rb    = rb;
>  	handle->event = event;
> @@ -244,6 +247,8 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
>  	INIT_LIST_HEAD(&rb->event_list);
>  	spin_lock_init(&rb->event_lock);
>  	init_irq_work(&rb->irq_work, rb_irq_work);
> +
> +	rb->paused = rb->nr_pages ? 0 : 1;
>  }
>  
>  static void ring_buffer_put_async(struct ring_buffer *rb)
> -- 
> 1.8.3.4