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    SubjectRe: [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

    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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