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

[Wangnan (F)]

On 2016/3/29 8:27, Alexei Starovoitov wrote:
> 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@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 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?

Good catch. Thank you!