Re: Performance counter API review was [patch] Performance Counters for Linux, v3

[Corey Ashford <cjashfor@linux.vnet.ibm.com>]

Andi Kleen wrote:
[snip]
> - Global tracing. Right now there seem to be two modi: per task and
> per CPU. But a common variant is global tracing of all CPUs. While this
> could be in theory done right now by attaching to each CPU
> this has the problem that it doesn't interact very well with CPU
> hot plug. The application would need to poll for additional/lost
> CPUs somehow and then re-attach to them (or detach). This would
> likely be quite clumsy and slow. It would be better if the kernel supported 
> that better.
> 
> Or alternative here is to do nothing and keep oprofile for that job
> (which it doesn't do that badly)
> 

This issue is of particular interest to us, from the IBM Power toolchain 
perspective.

Ingo, do you think it would be feasible to add an ability to open a 
single file descriptor that could give global counting (and sampling) on 
all CPU's?  I realize this would entail creating a context per cpu in 
the kernel.

How to present the count data back to user space is another issue.  For 
example, do you sum the counts of a particular event type across all 
CPUs or do you keep them separate, and have the user space app read them 
up per-cpu  (perhaps not knowing exactly which cpu they come from)?

I realize that perfmon doesn't have this ability either, it's currently 
per-cpu as well for global counting.

But it seems as long as you are going so far as providing a thread 
inheritance feature (which I assume uses a summing approach for 
providing counts back to user space), that this "pan-cpu" counting 
feature might not be too difficult to implement.  It sure would simplify 
the life of user space apps, as Andi said.

-- 
Regards,

- Corey

Corey Ashford
Software Engineer
IBM Linux Technology Center, Linux Toolchain
Beaverton, OR
503-578-3507
cjashfor@us.ibm.com