Re: [PATCH 1/2] perf tools: Add option to copy events when queueing

[Ingo Molnar <mingo@kernel.org>]

* 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