linux-kernel - Re: [PATCH 1/6] perf core: Introduce new ioctl options to pause and resume ring buffer

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Date:	Fri, 22 Jan 2016 11:36:44 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	peterz@...radead.org, acme@...nel.org,
	linux-kernel@...r.kernel.org, He Kuang <hekuang@...wei.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Zefan Li <lizefan@...wei.com>, pi3orama@....com
Subject: Re: [PATCH 1/6] perf core: Introduce new ioctl options to pause and
 resume ring buffer

On Fri, Jan 22, 2016 at 12:13:49PM +0000, Wang Nan wrote:
> 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.
> This patch is for supporting overwritable ring buffer with TAILSIZE
> selected.

TAILSIZE is dropped. pls adjust commit log.

> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> Cc: He Kuang <hekuang@...wei.com>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Brendan Gregg <brendan.d.gregg@...il.com>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Zefan Li <lizefan@...wei.com>
> Cc: pi3orama@....com
> ---
>  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..2c7f00c 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('$', 8, __u32)
> +#define PERF_EVENT_IOC_PAUSE_OUTPUT	_IO ('$', 9)

do you need to add '__u32' here, since boolean flag is passed to it?

> +	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;
> +	}