Messages in this thread | | | Date | Sat, 19 Jul 2008 08:11:26 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: Ftraced always polls once a minute even when no tracer is active |
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On Fri, 18 Jul 2008, Theodore Ts'o wrote:
> > I noticed with powertop that ftraced waks up once a second, even if no > tracer is active. Is there a reason why the kernel thread can't be > suspended in that case?
That's for the dynamic tracer. It is actually stopped when a tracer is active ;-)
CONFIG_FTRACE enables the "-pg" option in the kernel compile that enables the mcount profiler. This adds a call to a function called "mcount" at the start of every function. With a simple "ret" from mcount, this still adds up to a 18% overhead.
What the dynamic ftrace does is to have the mcount function call a "register_ip" function, that adds the caller into a hash table if it is not already there.
Once a second the "ftraced" kernel thread wakes up and checks to see if there are any new functions that have been called and recorded. If there has been, it calls kstop_machine and changes the call to mcount into a nop. This keeps the system to a 0% overhead (well, probably not 0% but it is not measurable ;-).
I should add code to stop the kthread when the system is low. I notice that the thread still wakes up but goes to sleep even when ftraced_enabled is zero.
I'm currently traveling and I'm trying to finish up the ftrace tutorial for OLS. I'll have to look more into this when I get back.
Thanks,
-- Steve
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