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Date: Wed, 1 Jul 2009 13:06:20 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: andi@firstfloor.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][RFC] Adding information of counts processes acquired
	how many spinlocks to schedstat
Message-ID: <20090701110620.GB15958@elte.hu>
References: <87hbxwj1k3.fsf@basil.nowhere.org> <20090701.174226.419764642024067218.mitake@dcl.info.waseda.ac.jp> <20090701090749.GA13535@elte.hu> <20090701.184232.131509470674490734.mitake@dcl.info.waseda.ac.jp>
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* Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp> wrote:

> From: Ingo Molnar <mingo@elte.hu>
> Subject: Re: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat
> Date: Wed, 1 Jul 2009 11:07:49 +0200
> 
> > 
> > * Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp> wrote:
> > 
> > > From: Andi Kleen <andi@firstfloor.org>
> > > Subject: Re: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat
> > > Date: Wed, 01 Jul 2009 09:38:04 +0200
> > > 
> > > > Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp> writes:
> > > > 
> > > > > Hi,
> > > > >
> > > > > I wrote a test patch which add information of counts processes acquired how many spinlocks to schedstat.
> > > > > After applied this patch, /proc/<PID>/sched will change like this,
> > > > 
> > > > The problem is that spinlocks are very common and schedstats is 
> > > > enabled commonly in production kernels. You would need to 
> > > > demonstrate that such a change doesn't have significant 
> > > > performance impact. For me it looks like it has.
> > > 
> > > I agree with your opinion about performance impact.
> > > I thought this will make no problem,
> > > because schedstat is categorized as "Kernel hacking" section.
> > > But according to you, many production kernels enable it
> > > so my patch will make widespread performance degradation.
> > > I didn't know that, sorry.
> > 
> > His arguments are bogus: both lockstat and perfcounters are optional 
> > (and default off), and the sw counter can be made near zero cost 
> > even if both perfcounters and lockstat is enabled. Also, sw counters 
> > are generally per CPU, etc. so not a performance issue.
> > 
> > The only (small) overhead will be when the lock-acquire sw counter 
> > is actively enabled because you run 'perf stat -e lock-acquire' - 
> > but that is expected and inherent in pretty much any kind of 
> > instrumentation.
> > 
> > The feature you are working on has the chance to be a very useful 
> > and popular piece of instrumentation. Being able to tell the lock 
> > acquire stats on a per task, per workload, per CPU or system-wide 
> > basis is a unique capability no other tool can offer right now.
> > 
> > Andi is often trolling perfcounters related (and other) threads, 
> > please dont let yourself be deterred by that and feel free to ignore 
> > him.
> OK, at least it is truth that
> counter in perfcounters makes only valid overhead.
> 
> And I have a question,
> I tried to build perf, but I got a build error,
> 
> util/symbol.c: In function ‘dso__load_sym’:
> util/symbol.c:466: error: ‘ELF_C_READ_MMAP’ undeclared (first use in this function)
> util/symbol.c:466: error: (Each undeclared identifier is reported only once
> util/symbol.c:466: error: for each function it appears in.)
> 
> I used this libelf,
> http://www.mr511.de/software/english.html
> but constant ELF_C_READ_MMAP is not provided...
> 
> which "libelf" should I use?
> It seems that there are some libelf implementations.

I use the elfutils-libelf* packages:

 elfutils-libelf-devel-static-0.141-1.fc10.i386
 elfutils-0.141-1.fc10.i386
 elfutils-libelf-0.141-1.fc10.i386
 elfutils-libs-0.141-1.fc10.i386
 elfutils-libelf-devel-0.141-1.fc10.i386

do they work fine or you?

	Ingo
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