Also send this to the perfmon mailing list. From: Gary Mohr Sent: Wednesday, April 09, 2014 4:56 PM To: Stephane Eranian Cc: Vince Weaver; Philip Mucci; Heike McCraw; Michel Brown Subject: Proposed enhancement to libpfm4.
Hi Stephane, There has been quite a bit of discussion in the PAPI community lately regarding ways to make the PAPI uncore component useful to existing PAPI applications. A short description of the problem: The kernel requires a cpu number to be provided on the open when setting up to count uncore events (used by kernel to pick the package/socket to count). PAPI currently provides a way to set a cpu number but it requires a call to PAPI_set_opt which existing papi applications that are currently used with core events almost never use. This means that existing PAPI applications cannot use uncore events without coding changes. Possible solution: Change the uncore event string to include information to specify the core number that should pass to the kernel for this event. PAPI applications normally get the event to use from a user or config file, so they would have access to uncore events if the user just adds a little extra information to the event string. Two approaches were considered: 1 -- The event name could be extended to include a package component. This would result in the event names being replicated once for each package on the system. 2 -- A new event mask could be added to provide the number of the core which should be passed to the kernel for the event. Since the SNBEP system already has 315 uncore events, replicating them for each package could lead to over 1200 different event names. The current list output for uncore events on this system produces 6,000+ lines of output. Replicating each event could drive that to about 24,000 lines of output. This makes the first approach less than desirable. A new mask for the uncore events could be added to identify which core number should be passed to the kernel. But this information is needed by PAPI and does not end up in the attribute structure built by libpfm4 and passed to the kernel by PAPI. This means that we would be introducing a mask that should be processed by PAPI and not libpfm4. The new mask approach would have no effect on the number of events and little or no effect on the list output. So it seemed to be the preferred approach. In addition during these discussions, it was felt that a small number of other PAPI attributes could also be handled with PAPI specific event masks rather than through independent API calls (as is required today). This encouraged looking for a general solution. Two different approaches for adding a mask have been considered: 1 -- Modify PAPI to prescan the event strings to remove and process the new mask. 2 - Enhance libpfm4 to allow event strings which contain masks it does know about. The first approach probably can be done but there is some concern that if PAPI prescreens and removes some of the event masks, it may remove masks that would have been meaningful to libpfm4. This would be undesirable but could be avoided with careful PAPI mask names. The idea behind the second approach is to add a feature to libpfm4 which would allow PAPI to pass an event string which contains some masks which libpfm4 may not understand. When this is done, libpfm4 would be able to return a table to the caller which contains the events libpfm4 did not recognize. When using this new feature, libpfm4 would not consider an unknown mask as an error. It would just return unprocessed masks to the caller and let the caller decide if those masks were valid. This provides PAPI with an easy way to extend the set of event masks an application can use. Of course when this new feature is not being used, libpfm4 would continue to behave exactly as it has in the past. I spent some time adding this feature to libpfm4 and now have it working. The end result is that I can now use papi_command_line to count uncore events without any changes to the application. A high level summary of what I did to libpfm4: I created two new libpfm4 functions which provide the same service as two existing functions but accept an additional calling argument. The additional calling argument is a pointer to a table where libpfm4 can store any unprocessed masks. The new functions are pfm_find_event_mask and pfm_get_os_event_encoding_mask. The current function names also still exist and just call the new functions passing a NULL pointer for the unprocessed masks table. Then the code in these new functions was changed to handle the case where it finds an unrecognized mask so it now behaves as described above. Attached you will find a patch file that contains the libpfm4 changes that I made (code is always more interesting than descriptions). I am hoping to persuade you that this code is worth putting into libpfm4 but in either case, I am interested in your views on the topic. There are still a few things in these patches that I think should be changed to make it more robust but if you are in agreement with this approach, I will gladly adjust it to meet expectations. I hope I did not bore you too much with details but I thought some of the background to explain why something in this area is needed was important. Thanks Gary
AddPapiSpecificMaskSupport_libpfm4.patch
Description: AddPapiSpecificMaskSupport_libpfm4.patch
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