On Fri, Jul 08, 2016 at 05:02:33PM +0200, Frederic Weisbecker wrote: > On Fri, Jul 08, 2016 at 09:29:29AM -0500, Josh Poimboeuf wrote: > > On Fri, Jul 08, 2016 at 12:08:19PM +0200, Ingo Molnar wrote: > > > > > > * Byungchul Park <byungchul.p...@lge.com> wrote: > > > > > > > On Mon, Jul 04, 2016 at 07:27:54PM +0900, Byungchul Park wrote: > > > > > I suggested this patch on https://lkml.org/lkml/2016/6/20/22. However, > > > > > I want to proceed saperately since it's somewhat independent from each > > > > > other. Frankly speaking, I want this patchset to be accepted at first > > > > > so > > > > > that the crossfeature can use this optimized save_stack_trace_norm() > > > > > which makes crossrelease work smoothly. > > > > > > > > What do you think about this way to improve it? > > > > > > I like both of your improvements, the speed up is impressive: > > > > > > [ 2.327597] save_stack_trace() takes 87114 ns > > > ... > > > [ 2.781694] save_stack_trace() takes 20044 ns > > > ... > > > [ 3.103264] save_stack_trace takes 3821 (sched_lock) > > > > > > Could you please also measure call graph recording (perf record -g), how > > > much > > > faster does it get with your patches and what are our remaining > > > performance hot > > > spots? > > > > > > Could you please merge your patches to the latest -tip tree, because this > > > commit I > > > merged earlier today: > > > > > > 81c2949f7fdc x86/dumpstack: Add show_stack_regs() and use it > > > > > > conflicts with your patches. (I'll push this commit out later today.) > > > > > > Also, could you please rename the _norm names to _fast or so, to signal > > > that this > > > is a faster but less reliable method to get a stack dump? Nobody knows > > > what > > > '_norm' means, but '_fast' is pretty self-explanatory. > > > > Hm, but is print_context_stack_bp() variant really less reliable? From > > what I can tell, its only differences vs print_context_stack() are: > > > > - It doesn't scan the stack for "guesses" (which are 'unreliable' and > > are ignored by the ops->address() callback anyway). > > > > - It stops if ops->address() returns an error (which in this case means > > the array is full anyway). > > > > - It stops if the address isn't a kernel text address. I think this > > shouldn't normally be possible unless there's some generated code like > > bpf on the stack. Maybe it could be slightly improved for this case. > > > > So instead of adding a new save_stack_trace_fast() variant, why don't we > > just modify the existing save_stack_trace() to use > > print_context_stack_bp()? > > I'm not sure this is a good idea. First of all if the kernel isn't built with > frame pointers, all you have is wild walk guesses. Also even if frame pointers > is built, the bp-non-validated "guesses" are important clues for debugging > because > they tell about previous calls that happened, or callbacks that were reffered > to by > the stack.
This was what I exactly intended to. > > There are several different users of save_stack_trace() in the kernel, we > can't > be sure that all of them are interested in dropping those guesses. > > So I'd rather advocate in favour of a new seperate helper. > > Thanks.
