Messages in this thread | | | From | Vineet Gupta <> | Subject | Re: dwarf unwinder question | Date | Mon, 23 Nov 2015 13:27:27 +0000 |
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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. > > Jan
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).
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