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

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Date:	Fri, 3 Oct 2014 06:34:21 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung.kim@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 1/2] perf tools: Add option to copy events when queueing


* Alexander Yarygin <yarygin@...ux.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?

Thanks,

	Ingo
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