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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 4/4] ring-buffer: only allocate buffers for online cpus
Cc: kosaki.motohiro@jp.fujitsu.com, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <peterz@infradead.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Wu Fengguang <fengguang.wu@intel.com>, Pierre Ossman <drzeus@drzeus.cx>,
       Pekka Paalanen <pq@iki.fi>, Steven Rostedt <srostedt@redhat.com>
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Date: Thu, 12 Mar 2009 16:31:44 +0900 (JST)
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> From: Steven Rostedt <srostedt@redhat.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 ;)