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SubjectRe: [PATCH 1/2] perf tools: Add option to copy events when queueing
On Fri, Oct 03, 2014 at 06:34:21AM +0200, Ingo Molnar wrote:
>
> * Alexander Yarygin <yarygin@linux.vnet.ibm.com> wrote:
>
> > When processing events the session code has an ordered samples
> > queue which is used to time-sort events coming in across
> > multiple mmaps. At a later point in time samples on the queue
> > are flushed up to some timestamp at which point the event is
> > actually processed.
> >
> > When analyzing events live (ie., record/analysis path in the
> > same command) there is a race that leads to corrupted events
> > and parse errors which cause perf to terminate. The problem is
> > that when the event is placed in the ordered samples queue it
> > is only a reference to the event which is really sitting in the
> > mmap buffer. Even though the event is queued for later
> > processing the mmap tail pointer is updated which indicates to
> > the kernel that the event has been processed. The race is
> > flushing the event from the queue before it gets overwritten by
> > some other event. For commands trying to process events live
> > (versus just writing to a file) and processing a high rate of
> > events this leads to parse failures and perf terminates.
> >
> > Examples hitting this problem are 'perf kvm stat live',
> > especially with nested VMs which generate 100,000+ traces per
> > second, and a command processing scheduling events with a high
> > rate of context switching -- e.g., running 'perf bench sched
> > pipe'.
> >
> > This patch offers live commands an option to copy the event
> > when it is placed in the ordered samples queue.
>
> What's the performance effect of this - i.e. by how much does CPU
> use increase due to copying the events?
>
> Wouldn't it be faster to fix this problem by updating the mmap
> tail pointer only once the event has truly been consumed?

Alexander mentioned he'd loose data, because of userspace processing
being to slow:
http://marc.info/?l=linux-kernel&m=141111652424818&w=2

jirka


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