Messages in this thread | | | Subject | Re: [PATCH 4/4] perf core: Add backward attribute to perf event | From | "Wangnan (F)" <> | Date | Tue, 29 Mar 2016 10:01:24 +0800 |
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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.
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