linux-kernel - Re: [PATCH] tracing/kmemtrace: normalize the raw tracer event to the unified tracing API

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Message-ID: <20081230084724.GA24637@elte.hu>
Date:	Tue, 30 Dec 2008 09:47:24 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] tracing/kmemtrace: normalize the raw tracer event to
	the unified tracing API


* Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro> wrote:

> On Tue, Dec 30, 2008 at 09:16:00AM +0100, Ingo Molnar wrote:
> > 3)
> > 
> > the most lowlevel (and hence most allocation-footprint sensitive) object 
> > to track would be the memory object itself. I think the best approach 
> > would be to do a static, limited size hash that could track up to N memory 
> > objects.
> > 
> > The advantage of such an approach is that it does not impact allocation 
> > patterns at all (besides the one-time allocation cost of the hash itself 
> > during tracer startup).
> 
> kmemtrace-user handles this by analysing offline :). I presume you could 
> get around this by discarding every hash collision in a well-sized 
> hashtable. The hashing algo in kmemtrace-user performs okay, considering 
> it fills the hashtable almost entirely, but I presume you're doing that 
> in-kernel and using other available code.

yeah - this is not a replacement for kmemtrace-user - analyzing raw trace 
events offline is still possible of course.

> > And this too would be driven from ftrace mainly - the SLAB code would 
> > only offer the alloc+free callbacks with the object IDs. [ and this 
> > means that we could detect memory leaks by looking at the hash table 
> > and print out the age of entries :-) ]
> 
> Some time ago I dropped timestamps because they were not providing a 
> good way to reorder packets in userspace. We're currently relying on a 
> sequence number to do that. You could take that as 'age', but it's not 
> temporally-meaningful.

yeah - ftrace entries generally have a timestamp so it should be rather 
easy.

	Ingo
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