Re: [RFC] perf_events: support for uncore a.k.a. nest units

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

On 1/29/2010 1:52 AM, Peter Zijlstra wrote:
> On Thu, 2010-01-28 at 14:08 -0800, Corey Ashford wrote:
>
>> This is one of the reasons why I am leaning toward a /sys/devices-style data
>> structure; the kernel could easily build it based on the pmus that it discovers
>> (through whatever means), and the user can fairly easily choose a pmu from this
>> structure to open, and it's unambiguous to the kernel as to which pmu the user
>> really wants.
>
> Well, the dumb way is simply probing all of them and see who responds.

That can work, but it's still fuzzy to me how a user would relate a PMU address 
that he's encoded to some actual device in the system he's using.  How would he 
know that he's addressing the correct device (besides that the PMU type 
matches), given that we're likely to have hypervisors as middle-men.

> Another might be adding a pmu attribute (showing the pmu-id) to the
> existing sysfs topology layouts (system topology, pci, spu, are all
> already available in sysfs iirc).

So you'd read the id from the sysfs topology tree, and then pass that id to the 
interface?  That's an interesting approach that eliminates the need to pass a 
string pmu path to the kernel.

I like this idea, but I need to read more deeply about the topology entries to 
understand how they work.

>> I am not convinced that this is the right place to put the event info for each PMU.
>
> Right, I'm not at all sure the kernel wants to know about any events
> beyond those needed for pmu scheduling constraints and possible generic
> event maps.
>
> Clearly it needs to know about all software events, but I don't think we
> need nor want exhaustive hardware event lists in the kernel.
>
>>   >  But before we go there the perf core needs to be extended to deal with
>>   >  multiple hardware pmus, something which isn't too hard but we need to be
>>   >  careful not to bloat the normal code paths for these somewhat esoteric
>>   >  use cases.
>>   >
>>
>> Is this something you've looked into?  If so, what sort of issues have you
>> discovered?
>
> I've poked at it a little yes, while simply abstracting the current hw
> interface and making it a list of pmu's isn't hard at all, it does add
> overhead to a few key locations.
>
> Another aspect is event scheduling, you'd want to separate the event
> lists for the various pmus so that the RR thing works as expected, this
> again adds overhead because you now need to abstract out the event lists
> as well.
>
> The main fast path affected by both these things is the task switch
> event scheduling where you have to iterate all active events and their
> pmus.
>
> So while the abstraction itself isn't too hard, doing it so as to
> minimize the bloat on the key paths does make it interesting.
>

Interesting.

Thanks for your comments.

- Corey