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

Wangnan (F)
2016-03-29T02:02:54Z

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.