Messages in this thread | | | Subject | Re: [patch] sched: fix single-depth wchan output | From | Matt Mackall <> | Date | Thu, 06 Nov 2008 15:56:11 -0600 |
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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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