linux-kernel - Re: [RFC PATCH v4] Unified trace buffer

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Message-Id: <1222413507.16700.235.camel@lappy.programming.kicks-ass.net>
Date:	Fri, 26 Sep 2008 09:18:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH v4] Unified trace buffer

On Thu, 2008-09-25 at 23:20 -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
> > 
> > On Thu, 25 Sep 2008, Masami Hiramatsu wrote:
> > 
> > > Hi Steven,
> > > 
> > > Steven Rostedt wrote:
> > > > This version has been cleaned up a bit. I've been running it as
> > > > a back end to ftrace, and it has been handling pretty well.
> > > 
> > > Thank you for your great work.
> > > It seems good to me(especially, encapsulating events :)).
> > 
> > Thanks!
> > 
> > > 
> > > I have one request of enhancement.
> > > 
> > > > +static struct ring_buffer_per_cpu *
> > > > +ring_buffer_allocate_cpu_buffer(struct ring_buffer *buffer, int cpu)
> > > > +{
> > > [...]
> > > > +	cpu_buffer->pages = kzalloc_node(ALIGN(sizeof(void *) * pages,
> > > > +					       cache_line_size()), GFP_KERNEL,
> > > > +					 cpu_to_node(cpu));
> > > 
> > > Here, you are using a slab object for page managing array,
> > > the largest object size is 128KB(x86-64), so it can contain
> > > 16K pages = 64MB.
> > > 
> > > As I had improved relayfs, in some rare case(on 64bit arch),
> > > we'd like to use larger buffer than 64MB.
> > > 
> > > http://sourceware.org/ml/systemtap/2008-q2/msg00103.html
> > > 
> > > So, I think similar hack can be applicable.
> > > 
> > > Would it be acceptable for the next version?
> > 
> > I would like to avoid using vmalloc as much as possible, but I do see the 
> > limitation here. Here's my compromise.
> > 
> > Instead of using vmalloc if the page array is greater than one page, 
> > how about using vmalloc if the page array is greater than 
> > KMALLOC_MAX_SIZE?
> > 
> > This would let us keep the vmap area free unless we have no choice.
> > 
> > -- Steve
> > 
> 
> You could also fallback on a 2-level page array when buffer size is >
> 64MB. The cost is mainly a supplementary pointer dereference, but one
> more should not make sure a big difference overall.

I'm still not sure why we don't just link the pages using the page
frames, we don't need the random access, do we?

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