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

* 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.

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)

Hm?

Ingo


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