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

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Message-ID: <20131002121821.GE10099@krava.brq.redhat.com>
Date:	Wed, 2 Oct 2013 14:18:21 +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 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! ;-)

> 
> 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.
> 

SNIP

>  
>  	new->timestamp = timestamp;
>  	new->file_offset = file_offset;
> -	new->event = event;
> +
> +	if (s->copy_on_queue) {
> +		new->event = malloc(event->header.size);
> +		memcpy(new->event, event, event->header.size);

missing allocation error check

jirka
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