linux-kernel - Re: [PATCH 4/4] ring-buffer: only allocate buffers for online cpus

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Date:	Thu, 12 Mar 2009 09:38:39 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Pierre Ossman <drzeus@...eus.cx>, Pekka Paalanen <pq@....fi>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 4/4] ring-buffer: only allocate buffers for online cpus


On Thu, 12 Mar 2009, KOSAKI Motohiro wrote:

> > From: Steven Rostedt <srostedt@...hat.com>
> > 
> > Impact: save on memory
> > 
> > Currently, a ring buffer was allocated for each "possible_cpus". On
> > some systems, this is the same as NR_CPUS. Thus, if a system defined
> > NR_CPUS = 64 but it only had 1 CPU, we could have possibly 63 useless
> > ring buffers taking up space. With a default buffer of 3 megs, this
> > could be quite drastic.
> > 
> > This patch changes the ring buffer code to only allocate ring buffers
> > for online CPUs.  If a CPU goes off line, we do not free the buffer.
> > This is because the user may still have trace data in that buffer
> > that they would like to look at.
> > 
> > Perhaps in the future we could add code to delete a ring buffer if
> > the CPU is offline and the ring buffer becomes empty.
> 
> I don't like this patch.
> your [1/4] and [2/4] already solve Pierre's problem.
> 
> using online cpu (not possible cpu) increase performance overhead 
> and messiness.
> but nobody get benefit ;)

Well, the fact that you can have 15 buffers for non existent CPUs is a big 
benefit. And the overhead was only on the read side, not the write, and 
very limited in overhead for that matter.

But, looking at this, I realized I can get rid of all the 
"get_online_cpus". I originally had the CPU_DOWN_PREPARE remove the 
buffer. But I found it highly annoying during tests, that I lose my data 
when I brought down a CPU. Thus, I removed the code to free the buffer and 
replaced it with the comment explaining this.

The get_online_cpus is to prevent the race where we might remove a buffer. 
But since we do not do that anymore, those get_online_cpus are pretty 
useless.

I'll update the code in a bit.

-- Steve

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