linux-kernel - Re: dwarf unwinder question

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075F44D2E16@IN01WEMBXA.internal.synopsys.com>
Date:	Mon, 23 Nov 2015 13:27:27 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	arcml <linux-snps-arc@...ts.infradead.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: dwarf unwinder question

On Monday 23 November 2015 06:45 PM, Jan Beulich wrote:
>>>> On 23.11.15 at 14:03, <Vineet.Gupta1@...opsys.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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