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    SubjectRe: [patch] sched: fix single-depth wchan output

    * 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.

    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)



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