Messages in this thread | | | Subject | Re: [PATCH 0/3][RFC] trace_printk() using percpu buffers | From | Peter Zijlstra <> | Date | Mon, 10 Oct 2011 15:03:19 +0200 |
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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.
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