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SubjectRe: [patch] sched: fix single-depth wchan output
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On Thu, 2008-11-06 at 07:11 +0100, Ingo Molnar wrote:
> * Ken Chen <kenchen@google.com> wrote:
>
> > To get a meaningful /proc/<pid>/wchan, one is required to turn on
> > full frame pointer when compile kernel/sched.c on x86 arch. The
> > enabling of frame pointer applies to entire kernel/sched.c and
> > affects lots of other core scheduler functions that aren't related
> > to wchan's call stack unwind. This causes unnecessary expansion of
> > stack pointer push and pop on the stack for scheduler functions. To
> > cut down the cost of frame pointer push/pop, one can use compile
> > time config option 'single-depth wchan'. However, the
> > 'single-depth' option is broken on x86 due to lack of stack frame
> > marker and simple stack unwind doesn't work, i.e., wchan always
> > produces '0'.
> >
> > This patch adds call site location explicitly in thread_struct for
> > schedule() function so that get_wchan() can reliably get the data
> > and at the same time not to overly burden the entire kernel/sched.c
> > with frame pointer generation. The remove of frame pointer
> > dependency allows compiler to generate better and faster core
> > scheduler code on x86_64.
>
> hm, this adds overhead - and the thing is that WCHAN is rather
> uninformative to begin with (because it's a single dimension), so we
> should phase it out, not expand it.

WCHAN is a long-standing public interface and isn't a Linuxism. I don't
think phasing it out is a good idea.

> How about adding a /proc/<PID>/stacktrace file that gives us the stack
> trace of any task in the system? That would be useful for a number of
> other purposes as well, and about 100 times more useful than wchan.
> (often it would be more useful than sysrq-t dumps)

But this is a great idea, of course.

--
Mathematics is the supreme nostalgia of our time.



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