Messages in this thread | | | Subject | Re: [PATCH 0/3][RFC] trace_printk() using percpu buffers | From | Steven Rostedt <> | Date | Mon, 10 Oct 2011 08:31:48 -0400 |
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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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