Messages in this thread | | | Date | Fri, 3 Oct 2014 10:47:54 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/2] perf tools: Add option to copy events when queueing |
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* Jiri Olsa <jolsa@redhat.com> wrote:
> 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
So copying helps by allocating an essentially larger buffer, to hold all unprocessed events that user-space is too slow to process?
I guess it's a valid usecase.
Thanks,
Ingo
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