Messages in this thread | | | Date | Tue, 19 Apr 2011 21:16:27 +0000 | Subject | Re: [BUG] perf_event: sampling buffer format cannot handle multi-event sampling | From | Stephane Eranian <> |
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On Tue, Apr 19, 2011 at 7:22 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, 2011-04-19 at 19:10 +0000, Stephane Eranian wrote: >> Hi, >> >> I was looking at the sampling buffer format and I think there is an >> issue in case >> one samples on more than one event at a time and each event as a different >> sample_type value (i.e., what it wants to record in each sample). >> >> The sample_type is exported per-event by the API. In order to decode a sample >> (PERF_RECORD_SAMPLE) one has to know which event caused it. To identify >> the event, one has to include PERF_SAMPLE_ID in the sample_type. Each such >> id uniquely identifies an event. Assuming the tool keeps a mapping of event to >> ID, we can then reconstruct. This is what perf does. >> >> The problem is that to extract the event ID, one has to already parse the sample >> itself. That means, you need sample_type to extract the event ID. >> >> Thus, we have a catch 22 situation. >> >> Looking at the perf tool, it sort of works today simply because the >> same sample_type >> is applied to all events. >> >> To solve this, we could either: >> - add the event ID to the header for PERF_RECORD_SAMPLE >> - enforce event ID is systematically saved at the beginning on >> PERF_RECORD_SAMPLE >> >> None of those options is transparent, I am afraid, unless we introduce >> new record types. > > Or simply don't mix different sample_types in the same buffer? > Does the kernel disallow this when you merge the output via that ioctl()? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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