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

[Wangnan (F)]

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.