linux-kernel - Re: [PATCH 1/3] perf tools: Cache register accesses for unwind processing

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Date:	Wed, 30 Apr 2014 14:12:37 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Jean Pihet <jean.pihet@...aro.org>
Subject: Re: [PATCH 1/3] perf tools: Cache register accesses for unwind
 processing

On Tue, Apr 29, 2014 at 09:36:19AM +0900, Namhyung Kim wrote:
> On Mon, 28 Apr 2014 15:24:20 +0200, Jiri Olsa wrote:
> > On Mon, Apr 28, 2014 at 10:02:55PM +0900, Namhyung Kim wrote:
> >> Hi Jiri,
> >> 
> >> 2014-04-28 (월), 11:48 +0200, Jiri Olsa:
> >> > On Sun, Apr 27, 2014 at 11:29:21PM +0900, Namhyung Kim wrote:
> >> > > Hi Jiri,
> >> > > 
> >> > > 2014-04-17 (목), 19:39 +0200, Jiri Olsa:
> >> > > > Caching registers value into an array. Got about 4% speed up
> >> > > > of perf_reg_value function for report command processing
> >> > > > dwarf unwind stacks.
> >> > > 
> >> > > I'm not familiar with the code base, so probably silly questions:  Where
> >> > > does the speed up come from?  IOW I don't know what's the difference
> >> > > between the regs->regs and regs->cached_regs.  And does the cached_regs
> >> > > contain correct values of registers for each frame?
> >> > 
> >> > the current way register's value is accessed is to get its
> >> > index in the sample's regs array.. based on register's id
> >> > and the registers mask
> >> > 
> >> > so each time you want register value you traverse the registers
> >> > mask and count reg's index for the sample regs array
> >> > 
> >> > this patch does this only once for each register (at the time it's
> >> > first accessed) and cache its value in the array (cache_regs). The
> >> > cache_mask is used to identify which regs are already cached.
> >> 
> >> That means it'll get the same value everytime it accesses a register in
> >> frames in a sample?
> >
> > right..
> 
> Hmm.. I thought it'd be changed somehow as it unwinds frames.

nope, it's just sample's user space registers values from the
time sample was taken

both libunwind and libdw unwinders keep the registers state
through the frames unwinding internally

jirka
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