Messages in this thread | | | Date | Mon, 23 Nov 2015 06:42:34 -0700 | From | "Jan Beulich" <> | Subject | Re: dwarf unwinder question |
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>>> On 23.11.15 at 14:27, <Vineet.Gupta1@synopsys.com> wrote: > On Monday 23 November 2015 06:45 PM, Jan Beulich wrote: >>>>> On 23.11.15 at 14:03, <Vineet.Gupta1@synopsys.com> wrote: >>> I was wondering if u could answer a question in that respect: >>> arch/arc/kernel/unwind.c >>> >>> If the binary search for a PC fails, it resorts to linear search, which for >>> our >>> case was taking 3 million cycles (vs. normal ~2000). >>> Do you remember why this linear search step was needed - after all the binary >>> lookup table is created out of early parsing of the same data. >>> >>> The fail scenario is for hand asm symbols lacking gcc generated dwarf info >>> and we >>> don't have yet the CFI pseudo ops support in assembler. >>> I can fix memset etc to have empty dwarf info, still unwinder needs this >>> fixing. >>> >>> In case of perf, an overflow interrupt in hand optimized memset leads into >>> the >>> unwinder slow path linear search which causes RCU stalls and such. >>> I'm going to remove it but was wondering if u could provide some historic >>> background. >> Iirc there was no binary lookup at all originally. When it got added, >> it seemed odd to remove the linear lookup altogether (want to keep >> it at least for the case where the binary lookup table couldn't be >> built for whatever reason), and code structure seemed most >> reasonable to simply do one after the other instead of just either. >> I'm pretty sure the linear lookup could be skipped if you're sure the >> binary lookup table is correct and complete. > > Thx for quick reply. I'll remove the linear search as part of many other > tweaks to > speed it up - we can elide a lot of general dwarf checks / rechecks - given > that > it is used only for kernel unwinding (not user space).
Don't tell Linus if you're removing any checks...
Jan
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