linux-kernel - Re: [RFC] Unified Ring Buffer (Next Generation)

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Message-ID: <20100520064139.GA15946@basil.fritz.box>
Date:	Thu, 20 May 2010 08:41:40 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC] Unified Ring Buffer (Next Generation)

> The plan here is to create a ring buffer that supports per-buffer instance
> "flags" that specify what must be supported: e.g. either splice() or mmap(),
> global vs per-cpu buffers, etc.

And you plan to test all those flags in the hot path? 

> The new implementation I propose lessens the complexity level, presents clear
> abstractions to deal with that complexity, and comes with a formal proof of
> correctness, all of which I think is really very important to give a good level
> of insurance that the ring buffer works as expected.

Any simplifcation for the ftrace buffer would be a good thing IMHO.

> > For debugging kernels etc. with tracing that's not that big an issue, but
> > I think it's a problem for "non debugging" use. After all Linux
> > still has the goal to be at least configurable as a low footprint operating
> > system.
> 
> My implementation, at the moment, has 50% less lines of code and is 25% smaller
> in object size than the current ring buffer.

Good.

> 
> But all in all, I think users needing _something_ to perform system-wide tracing
> shout a lot louder than users who need to save a few bytes. So let's try to get
> something good in first, while keeping an eye on the object size, and if it
> happens to be too large for some users, then they can always implement a
> slower and less efficient ring_buffer_tiny.c if they feel like it.

They don't need to, they already have kfifo.

> I totally agree with you. This is in good part why I spent a large part of 2009
> writing papers explaining my ring buffer, doing Promela models and formal proofs
> of correctness. I think after all that work, the abstractions I will use will be
> much easier to grap by anyone willing to do a bit of reading.

Writing papers is not a replacement for simple maintainable code.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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