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Date: Fri, 26 Sep 2008 17:05:26 -0700
From: Mike Travis <travis@sgi.com>
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To: Steven Rostedt <rostedt@goodmis.org>
CC: Peter Zijlstra <peterz@infradead.org>, LKML <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       Andrew Morton <akpm@linux-foundation.org>, prasad@linux.vnet.ibm.com,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Mathieu Desnoyers <compudj@krystal.dyndns.org>,
       "Frank Ch. Eigler" <fche@redhat.com>, David Wilder <dwilder@us.ibm.com>,
       hch@lst.de, Martin Bligh <mbligh@google.com>,
       Christoph Hellwig <hch@infradead.org>,
       Masami Hiramatsu <mhiramat@redhat.com>,
       Steven Rostedt <srostedt@redhat.com>,
       Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Subject: Re: [PATCH v6] Unified trace buffer
References: <20080925185154.230259579@goodmis.org>  <20080925185236.244343232@goodmis.org>  <alpine.DEB.1.10.0809252101080.8124@gandalf.stny.rr.com>  <48DC406D.1050508@redhat.com>  <alpine.DEB.1.10.0809261245420.21618@gandalf.stny.rr.com>  <alpine.DEB.1.10.0809261349400.21618@gandalf.stny.rr.com> <1222456474.16700.289.camel@lappy.programming.kicks-ass.net> <48DD61F4.3000704@sgi.com> <alpine.DEB.1.10.0809261956000.21618@gandalf.stny.rr.com>
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Steven Rostedt wrote:
> On Fri, 26 Sep 2008, Mike Travis wrote:
> 
>> Peter Zijlstra wrote:
>>> On Fri, 2008-09-26 at 14:05 -0400, Steven Rostedt wrote:
>>>> +struct ring_buffer {
>>>> +       unsigned long           size;
>>>> +       unsigned                pages;
>>>> +       unsigned                flags;
>>>> +       int                     cpus;
>>>> +       atomic_t                record_disabled;
>>>> +
>>>> +       struct mutex            mutex;
>>>> +
>>>> +       /* FIXME: this should be online CPUS */
>>>> +       struct ring_buffer_per_cpu *buffers[NR_CPUS];
>>> actually nr_possible makes sense, and you might consider always
>>> allocating buffers (and keeping them for offlined cpus) to avoid massive
>>> allocations/frees cpu-hotplug events.
>>>
>>> Mike Travis has been going over the kernel removing constructs like
>>> this, and replacing them with dynamically allocated arrays of
>>> nr_possible.
>>>
>>>> +};
>> The other thing to consider is using a percpu variable.
> 
> This structure is allocated on request.
> 
> -- Steve


Ahh, then it would need the yet to be added cpu_alloc() from Christoph.

Your best bet then is to allocate based on nr_cpu_ids.

Cheers,
Mike