linux-kernel - Re: [PATCH 5/6] perf_counter: add more context information

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Date:	Thu, 02 Apr 2009 13:46:10 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] perf_counter: add more context information

On Thu, 2009-04-02 at 13:36 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > Put in counts to tell which ips belong to what context.
> > 
> >   -----
> >    | |  hv
> >    | --
> > nr | |  kernel
> >    | --
> >    | |  user
> >   -----
> 
> btw., i have an observation about the format:
> 
> > -#define MAX_STACK_DEPTH		255
> > +#define MAX_STACK_DEPTH		254
> >  
> >  struct perf_callchain_entry {
> > -	u64	nr;
> > +	u32	nr, hv, kernel, user;
> >  	u64	ip[MAX_STACK_DEPTH];
> >  };
> 
> For the special case of signal notifications, if the signal is 
> delivered immediately to the same task that raised it (pid=0), the 
> call chain is actually a still meaningful one: it is the stack that 
> is below the currently executing signal handler context.
> 
> Wouldnt it make sense to record the full stack frame for that case, 
> to allow walking/unwinding of the stack? Or can user-space do that 
> just fine, based on its own signal context?

I think it can do that just fine or even better than we can -- userspace
having access to a full dwarf2 unwinder and such.

> We are going to hard-code the "call-chain is a series of IPs, 
> nothing else" model, and i'd like to make sure it's future-proof :)

I think it should be, function return addresses are the primary piece of
information here.

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