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SubjectRe: BUG: Function graph tracer hang

* Steven Rostedt <rostedt@goodmis.org> wrote:

>
> On Tue, 28 Apr 2009, Frederic Weisbecker wrote:
>
> >
> > Note that the branch profiler does that:
> >
> > ______f.miss_hit[______r]++;
> >
> > Which is a read + write on the cacheline.
> > If each "if" are profiled in the timer interrupt, we can
> > have the cachelines doing a ping-pong of dirtifying since the above
> > variable is shared.
> >
> > Then the timer interrupt becomes slower. The function graph tracer itself makes
> > it slower.
> > Moreover it is traced itself. So not only the "if" in code are traced, but also
> > each "if" processed by the function graph tracer on function calls and returns.
> >
> > Which means a fair amount of cacheline dirtifying.
> >
> > Then if the timer interrupt is slowed, and we have a lot of them (1000 Hz),
> > the system spends all of its time inside it.
> >
> > At least we need the branch tracing to be done per cpu, I guess.
>
> This can be done by basically reimplementing what percpu does.
> This is because the data is saved off in its own section at every
> if statement. We could copy that section per cpu and add code to
> the incrementors to add only to their own CPU buffers.

Why not just make these variables regular percpu constructs?

> This is low in my priority of things to do, but I'll at least add
> it to my "to do" list.

I think we should mark the branch tracer as CONFIG_BROKEN - there's
been too many problems with it. Thoughts?

Ingo


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