linux-kernel - Re: [PATCH v6] Unified trace buffer

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Date:	Fri, 26 Sep 2008 15:52:14 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Martin Bligh <mbligh@...gle.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v6] Unified trace buffer


On Fri, 26 Sep 2008, Martin Bligh wrote:

> On Fri, Sep 26, 2008 at 11:59 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Fri, 2008-09-26 at 14:05 -0400, Steven Rostedt wrote:
> >
> >> +struct buffer_page {
> >> +     u64             time_stamp;
> >> +     unsigned char   body[];
> >> +};
> >> +
> >> +#define BUF_PAGE_SIZE (PAGE_SIZE - sizeof(u64))
> >
> > Since you're already using the page frame, you can stick this per page
> > timestamp in there as well, and get the full page for data.
> >
> > You can either use a struct page overlay like slob does, or add a u64 in
> > the union that contains struct {private, mapping}.
> 
> What did you guys think of Mathieu's idea of sticking the buffer length
> in the header here, rather than using padding events? Seemed cleaner
> to me.

Actually I like the padding. This way when I move the event pointer 
forward, I only need to compare it to a constant (PAGE_SIZE), or test to 
see if the event is padding.  Placing this into the buffer page, I will 
have to always compare it to that pointer.

But I guess I could change it to that if needed. That doesn't affect the 
API, as it is only internal.

I'm almost done with v7, perhaps I might try that with v8 to see if I like 
it better.

-- Steve

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