On Fri, Oct 03, 2014 at 06:34:21AM +0200, Ingo Molnar wrote: > > * Alexander Yarygin <yary...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
