linux-kernel - Re: dwarf unwinder question

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Message-Id: <56531F8502000078000B7D52@prv-mh.provo.novell.com>
Date:	Mon, 23 Nov 2015 06:15:33 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Vineet Gupta" <Vineet.Gupta1@...opsys.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 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

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