Date:	Mon, 17 Nov 2008 16:16:23 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	eranian@...il.com
CC:	Andi Kleen <andi@...stfloor.org>,
	Mikael Pettersson <mikpe@...uu.se>,
	Robert Richter <robert.richter@....com>,
	oprofile-list@...ts.sf.net, Ingo Molnar <mingo@...e.hu>,
	Jiri Kosina <jkosina@...e.cz>, Jiri Benc <jbenc@...e.cz>,
	Vilem Marsik <vmarsik@...e.cz>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org
Subject: Re: Oprofile : need to adjust PC by 16 bytes

stephane eranian a écrit :
> Hello,
> 
> I have not seen the beginning of that discussion so my comments may be
> slightly off.
> It seems Eric has problems with accuracy of instruction addresses when
> sampling with the PMU.
> 
> This is an inherent limitation of the PMU. It can be mitigated but not
> completely eliminated. The core
> issue is that it takes several cycles between the moment a counter
> overflows and posting of the PMU
> interrupt. During that time, the CPU keeps on executing instructions.
> The interrupt IP you get, reflects
> the place you were when it triggered. That can be far away from where
> it was posted and where the
> counter actually overflowed. Of course, if you are stalled that
> distance is usually 0 or off by a small
> number of instructions. But it can be very large when overflow happens
> during a kernel critical section
> where interrupts are off. There is nothing SW can do about all of this.
> 
> Andi mentioned PEBS. I don't know if you are familiar with what it
> does. Let me summarize. This is
> a hardware/microcode feature which implements a hardware-managed
> buffer where samples are
> stored. The OS points the CPU to a memory region where PMU samples are
> stored. No PMU
> interrupt is generated until the buffer becomes full. That part
> addresses some of the overhead
> associated with interrupt-based sampling. Unfortunately, PEBS  does
> not point to the instruction where
> the counter overflowed, it will still be a few instructions off. But
> this time, you get the machine state at the
> last retired instruction. Furthermore, PEBS can record samples while
> in kernel critical sections. A limitation
> of PEBS is that it does not work with all the PMU events. Only a
> handful are available.
> 
> As for perfmon, if you pull from the perfmon2 GIT tree, this should
> work. Don't know what happen in
> you case.
> 
> Perfmon and the pfmon can do simple counting or also collect profiles.
> 
> $ pfmon date
> 
> Counts cycles at the user level only for the process date
> 
> $ pfmon --system-wide -t10
> 
> Counts elapsed cycles at user level for all CPU for 10s. Results are per-cpu
> 
> $ pfmon --long-smpl-periods=240000 date
> 
> Collect a flat profile of process date. Period is 240,000 elapsed user cycle
> 
> $ pfmon --system-wide --long-smpl-periods=240000 -t 10
> 
> Collect a flat profile on each online CPU during 10s. Period is
> 240,000 user elapsed cycles. Results are per-cpu
> 
> You have a lot  more examples on the perfmon web site, Following the
> documentation and pfmon users' guide.
> 
> Perfmon/pfmon can use PEBS on Intel Core processors. First step is to
> insert the kernel module for it:
>    # modprobe perfmon_pebs_core_smpl
> 
> Then use pfmon, we use instruction_retired because elapsed cycles does
> not support PEBS:
> 
>    $ pfmon --smpl-module=pebs -einstructions_retired
> --long-smpl-periods=120000 date
> 
> Hope this helps.
> 

Sure it does ! Thanks a lot Stephane, I am going to try all this stuff.


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