Posted Sep 25, 2013 3:19 UTC (Wed) by deater (subscriber, #11746)
In reply to: Good one by ncm
Parent article: A perf ABI fix
The perf_event interface is full of bitfields for reasons I don't fully understand.
To make things more fun, there are proposals in the works to export the bit offsets in these bitfields (specifically the ones in struct perf_event_attr) via /sys so that the kernel can export event configs to the perf tool more "efficiently". I personally think this will only end in tears. Especially once endianess is factored in.
Posted Sep 25, 2013 3:41 UTC (Wed) by deater (subscriber, #11746)
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Also I should probably disclose that I'm the Vince Weaver who apparently has become famous for being grumpy about the perf_event ABI.
In this case I was grumpy because the initial Changelog for the structure re-arrangement did not mention anything at all about the ABI implications or the bit overlap.
It was only by luck that I noticed this issue, because I had updated the perf_event.h header in my perf_event_tests testsuite to 3.12-rc1 but had rebooted back to 3.11 for other reasons. If I hadn't done that it's likely no one would have noticed this issue until after the 3.12 release.
Not that it matters a lot though, as I'm possibly the only person in the world actually using RDPMC for anything right now. It's used by the High Performance Computing people for low-latency self monitoring, but the perf tool doesn't use the interface at all.
Good one
Posted Sep 25, 2013 9:19 UTC (Wed) by luto (subscriber, #39314)
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Are you using some library or doing this directly? I'd like to do the same thing, but the API seems to be (intentionally) poorly documented.
Good one
Posted Sep 25, 2013 13:39 UTC (Wed) by deater (subscriber, #11746)
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> Are you using some library or doing this directly? I'd like to do the same
> thing, but the API seems to be (intentionally) poorly documented.
I'm currently doing the RDPMC accesses directly. The eventual goal is to have the PAPI performance library use the interface; there are overhead issues with the interface I was dealing with first (sometimes it is slower to use RDPMC than to just use the read() syscall, for reasons that took me a long time to figure out. Thankfully there are workarounds).
In any case yes, the documentation is awful. I wrote the perf_event_open() manpage in an attempt to address this. I've been working on updating the RDPMC part of that recently, although had to spend time trying to sanely document this ABI issue instead.
Don't go by the example RDPMC code in perf_event.h, it's out of date and possibly never really worked. I've been meaning to send a patch to fix that.
Good one
Posted Sep 25, 2013 19:20 UTC (Wed) by mathstuf (subscriber, #69389)
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> Don't go by the example RDPMC code in perf_event.h, it's out of date and possibly never really worked. I've been meaning to send a patch to fix that.
Could a patch which replaces it with "TODO: Add an example" (or similar) be pushed for 3.12 at least? If there's anything worse than no documentation, it's bad documentation.
