Re: [PATCH 4/4] perf core: Add backward attribute to perf event

["Wangnan (F)" <wangnan0@huawei.com>]

On 2016/3/28 14:41, Wang Nan wrote:

[SNIP]

>
> To prevent this problem, we need to find a way to ensure the ring buffer
> is stable during reading. ioctl(PERF_EVENT_IOC_PAUSE_OUTPUT) is
> suggested because its overhead is lower than
> ioctl(PERF_EVENT_IOC_ENABLE).
>

Add comment:

By carefully verifying 'header' pointer, reader can avoid pausing the
ring-buffer. For example:

     /* A union of all possible events */
     union perf_event event;

     p = head = perf_mmap__read_head();
     while (true) {
         /* copy header of next event */
         fetch(&event.header, p, sizeof(event.header));

         /* read 'head' pointer */
         head = perf_mmap__read_head();

         /* check overwritten: is the header good? */
         if (!verify(sizeof(event.header), p, head))
             break;

         /* copy the whole event */
         fetch(&event, p, event.header.size);

         /* read 'head' pointer again */
         head = perf_mmap__read_head();

         /* is the whole event good? */
         if (!verify(event.header.size, p, head))
             break;
         p += event.header.size;
     }

However, the overhead is high because:

  a) In-place decoding is unsafe. Copy-verifying-decode is required.
  b) Fetching 'head' pointer requires additional synchronization.