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    SubjectRe: [patch] sched: fix single-depth wchan output
    On Thu, 2008-11-06 at 07:11 +0100, Ingo Molnar wrote:
    > * Ken Chen <> 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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