Subject: Re: Sampling instruction pointer on PPC

From: Victor Jimenez <[email-masked]>
Date: Fri, 02 Mar 2012 20:21:35 +0100

Hello Carl,

On 03/01/2012 06:45 PM, Carl E. Love wrote:
> Victor:
>
> The performance counter tools perf and OProfile capture the value of the
> instruction pointer at the time an interrupt occurs. The file
> arch/powerpc/oprofile/op_model_power4.c contains the OProfile interrupt
> handler used for Power 4, 5, 6, and 7.  When the performance counters
> overflow, the instruction pointer (Program Counter) is stored in the
> SIAR register.  The perf and OProfile interrupt handlers store the
> instruction address in their samples.  These tools then use the
> addresses to create a histogram of where the processor was at the time
> of the interrupt.
>
> I am guessing you are hoping to collect something more like an raw
> sequence of addresses trying to trace where the processor was in time.
That is exactly what I want to do. Basically I want to use IP sampling 
for detecting and keeping application phase changes.
> I am not aware of a way to read the value of the instruction pointer
> directly on the fly.  But you could possibly emulate it by setting a
> program counter count value to 2^31 -1, enable the counter to count
> cycles, then read the SIAR value.  You could do this each time you want
> to take a sample.  It would require a couple reads/writes to registers.
> Note, the counter generates the interrupt to store the instruction
> pointer into the SIAR register when the most significant bit of the 32
> bit HW count value changes from 0 to 1.  You will need to then clear the
> interrupt, actually I would have to double check but I think the default
> interrupt handler that is enabled with Perf and OProfile are not running
> will do that quietly for you.
Indeed this seems like it may do the trick. Just one question; from your 
description I assume that there is no support in the current 
implementation of perf (e.g., I know perf_event_read_value() is used to 
read the value of an event, but I cannot find any equivalent function 
that can write to a counter). I guess that means I need to implement 
that in the kernel myself, right?
> Not sure that helps.
>               Carl Love
Thank you!
Victor
>
>
>
> On Thu, 2012-03-01 at 18:08 +0100, Victor Jimenez wrote:
>> I am trying to sample instruction pointer along time on a Power7 system.
>> I know that there are accurate mechanisms to do so in Intel processors
>> (e.g., PEBS and Branch Trace Store).
>>
>> Is it possible to do something similar in Power7? Will the samples be
>> accurate? I am worried that significant delays (skids) may appear.
>>
>> Thank you,
>> Victor
>>
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-- 
------------------------------------------------------------------------

      Victor Jimenez Perez
      Barcelona Supercomputing Center
      Centro Nacional de Supercomputacion
      WWW: http://www.bsc.es         Tel: +34-934137167
      e-mail: [email-masked]

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