linux-kernel - Re: [PATCH] perf session: Add option to copy events when queueing

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Date:	Wed, 2 Oct 2013 14:38:08 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	David Ahern <dsahern@...il.com>
Cc:	acme@...stprotocols.net, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf session: Add option to copy events when queueing

On Wed, Oct 02, 2013 at 02:18:21PM +0200, Jiri Olsa wrote:
> On Fri, Sep 06, 2013 at 01:37:01PM -0600, David Ahern 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.
> 
> hi,
> 'perf top -G dwarf' hit this due to longer callchains
> processing your patch fixed that, thanks! ;-)

ugh... now looking closer on top processing, it does not
seem to use queues ;-\ must be something else then..


> 
> > 
> > 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.

So I guess you have some other patch that actually sets
session::copy_on_queue?

jirka
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