Gary,
I am trying to understand the underpinning here. You are saying there is no
way to pass a CPU to the PAPI call
to pin an uncore event to a particular socket.
First, uncore events are system-wide only events. This is why you need to
pass a CPU number (as a substitute
for a socket number). Second, the kernel always exports a list of CPUs to
monitor for each uncore PMU. It is
located in /sys/device/uncore_xxx/cpumask.
I don't really like the libpfm4 changes you are proposing. They do not make
sense to me because you are trying
to work around a limitation of PAPI by modifying libpfm4.
My understanding is that PAPI is not designed to handle system-wide events.
System-wide events require a CPU
number. So why not extend PAPI to handle this instead so it would work with
or without libpfm4? I understand it
would break existing tools, but then those tools are not ready to cope with
CPU or socket-level measurements, maybe.
On Thu, Apr 10, 2014 at 5:40 PM, Gary Mohr <gary.m...@bull.com> wrote:
> 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
>
>
>
>
>
>
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