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

[Wangnan (F)]

Hi Peter,

 Patch 10/46 to 14/46 were sent separately to you and modified
follow your suggestion. Do you have further comment on it?

Thank you.

On 2016/2/29 23:39, Arnaldo Carvalho de Melo wrote:

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 <wangn...@huawei.com>
Cc: He Kuang <heku...@huawei.com>
Cc: Alexei Starovoitov <a...@kernel.org>
Cc: Arnaldo Carvalho de Melo <a...@redhat.com>
Cc: Brendan Gregg <brendan.d.gr...@gmail.com>
Cc: Jiri Olsa <jo...@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu...@hitachi.com>
Cc: Namhyung Kim <namhy...@kernel.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Zefan Li <lize...@huawei.com>
Cc: pi3or...@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 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