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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2011-10-10 at 08:31 -0400, Steven Rostedt wrote:

> > > 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.

But its also a massive scalability fail. There's simply no sane reason
to keep the shared buffer trace_printk() implementation.

> > > 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.

I guess it doesn't really matter that much..

> > > 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 :)

Ah, right :-)

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

Well, if all we have is the per-cpu option I'm fine either way.