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

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Message-ID: <20090402113643.GF10828@elte.hu>
Date:	Thu, 2 Apr 2009 13:36:43 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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


* 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?

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 :)

	Ingo
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