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

[Wangnan (F)]

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.