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

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Date:	Fri, 23 Sep 2011 07:16:45 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.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 13:07 +0200, Peter Zijlstra wrote:
> 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? #$%@^%@@$%@

It is NMI safe, always was (I use it there too). It has a percpu
recursion detection (always has), thus if an NMI interrupts a current
trace_printk(), the NMI trace_printk() will not print. I could add an
NMI buffer to allow NMIs to print, but so far, we don't usually have
issues with trace_printk(). Heck, I'm not sure printk() wont cause
issues in NMIs. I think trace_printk() is still safer than printk.

> 
> 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'.

Not, having per cpu buffers still doesn't allow NMIs to interrupt
trace_printk(). Otherwise the NMI would just corrupt the current percpu
buffer.

-- Steve


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