Messages in this thread | | | Date | Thu, 6 Nov 2008 07:11:25 +0100 | From | Ingo Molnar <> | Subject | Re: [patch] sched: fix single-depth wchan output |
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* 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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