linux-kernel - Re: [PATCH 21/21] tracing: Add optional percpu buffers for trace

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Date:	Fri, 23 Sep 2011 13:07:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 21/21] tracing: Add optional percpu buffers for
 trace_printk()

On Fri, 2011-09-23 at 13:02 +0200, Peter Zijlstra wrote:
> On Thu, 2011-09-22 at 18:09 -0400, Steven Rostedt wrote:
> > 
> > Currently, trace_printk() uses a single buffer to write into
> > to calculate the size and format needed to save the trace. To
> > do this safely in an SMP environment, a spin_lock() is taken
> > to only allow one writer at a time to the buffer. But this could
> > also affect what is being traced, and add synchronization that
> > would not be there otherwise. 
> 
> so trace_printk() isn't NMI safe? #$%@^%@@$%@

better to make all of trace_printk() depend on that extra config, there
is absolutely 0 point in having a broken and fully serialized trace
'fail^wfeature'.
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