Re: [PATCH 1/4] perf core: Introduce new ioctl options to pause and resume ring buffer

Alexei Starovoitov
2016-03-29T00:27:36Z

On Mon, Mar 28, 2016 at 06:41:29AM +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
> 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.
>
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Cc: He Kuang <hekuang@huawei.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Zefan Li <lizefan@huawei.com>
> Cc: pi3orama@163.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..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 de24fbc..cb47da3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4341,6 +4341,19 @@ 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) {

should have been 'if (!rb)', right?