Re: PM_L2_LD_MISS events doesn't work

Hi Carl,

just a question... What does 'hypervisor' exactly mean here? Do you mean
a PowerKVM host or a PowerVM one? Or does it apply to both? And finally,
how does that work on a POWER8 machine that runs without virtualization?
Are the events still not accessible there?

Thanks,
Michael


On Fri, 2015-11-06 at 09:50 -0800, Carl E. Love wrote:
> Chencheng Ye:
> 
> I looked into the issue some more.  The libpfm is not an issue.  The
> name to event number lookup is working fine.  The PM_L2_LD_MISS event
> generates the event code 0x426084.  You can see the event code ocount is
> using if you use the option --verbose.  It will print something like:
> 
> ocount --verbose  -e PM_L2_LD_MISS ls
> Final event code is 426084
> Number of events passed is 1
> Exec args are: ls 
> 
> where the "Final event code" is printed in decimal.  
> 
> The event requires the cache selector bit to be set to be set to 1 in
> the processor performance counter setup register.  On Power 8, this
> register only writable by the hypervisor.  So the kernel must reject any
> event where the selector bit (bit 22) is not equal to 0.  So until they
> come up with an API so the kernel can issue a request to the hypervisor
> to write the bit, any event with bit 22 set to a 1 will be rejected.
> 
> This is an issue for OProfile and perf.  OProfile (operf and ocount) are
> actually using perf under the cover to setup and read the hardware
> performance counters.  The kernel code that does the rejection of the
> event is actually the perf kernel code for Power 8.  If you are really
> interested, you can see the code in  arch/powerpc/perf/power8-pmu.c.
> 
> There is a comment at about line 77 of the file that describes the
> register layout.  Then lines 327 to 337 is where the code is that
> actually checks that bit 22 is 1 and rejects the event if the bit is a
> 1. 
> 
> So, that said, we need to update the list of supported OProfile events
> to remove these until such time that the kernel is fixed.
> 
>               Carl Love
> 



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