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

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

On 2016/3/29 12:59, Alexei Starovoitov wrote:
> On Tue, Mar 29, 2016 at 10:01:24AM +0800, Wangnan (F) wrote:
>>
>> 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.
> Such trick may work, but pause is needed for more than stability
> of reading. When we collect the events into overwrite buffer
> we're waiting for some other trigger (like all cpu utilization
> spike or just one cpu running and all others are idle) and when
> it happens the buffer has valuable info from the past. At this
> point new events are no longer interesting and buffer should
> be paused, events read and unpaused until next trigger comes.

Agree. I just want to provide an alternative method.
I'm trying to finger out pausing is not mandatory
but highly recommended in man page and commit
messages.

Thank you.