linux-kernel - Re: [PATCH 0/3][RFC] trace_printk() using percpu buffers

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Date:	Mon, 10 Oct 2011 08:31:48 -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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 0/3][RFC] trace_printk() using percpu buffers

On Mon, 2011-10-10 at 13:04 +0200, Peter Zijlstra wrote:
> On Sat, 2011-10-08 at 13:02 -0400, Steven Rostedt wrote:
> > 
> > Peter,
> > 
> > You had issues with the previous version of my trace_printk() code.
> > I rewrote it to do the following.
> > 
> > By default, it still uses the single buffer protected by a spinlock
> > and an atomic (for NMIs). The NMI case can cause dropped prints if
> > the NMI happens while a trace_printk() is processing.
> 
> Why bother keeping that?

Because very few developers debug nmi's. printk is known not to work
there.

I still find it useful to have without having to switch on a config
option or kernel command line.

> 
> > When trace_printk_percpu is enabled, either via the trace options or
> > the kernel command line, then two sets of percpu buffers are made,
> > one for normal and irqs (interrupts are still disabled), and the other
> > is for NMIs. These can be added or removed at anytime.
> 
> So why not allocate 4, one for {task, softirq, irq, NMI} resp, then all
> you need to do is disable preemption.
> 
> depending on tracing/options/trace_printk ?

Preemption still needs to be disabled. But if you think that's better
than disabling interrupts, I could do that too.

> 
> > The last patch adds a CONFIG_TRACE_PRINTK_PERCPU that makes trace_printk()
> > permanently use two sets of per_cpu buffers, and these can not be
> > removed. This will give the least amount of overhead for trace_printk()
> > with the sacrifice of memory overhead. This is an option I could imagine
> > you would just set and forget about.
> 
> Is that one dereference really that expensive?

It's also a compare and jump, but I added this option for you :)

That way, you could set this option and forget about it.

-- Steve


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