Messages in this thread | | | Date | Thu, 11 Jul 2013 11:16:48 -0600 | From | David Ahern <> | Subject | Re: [PATCH V5 09/12] perf: make events stream always parsable |
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On 7/11/13 9:43 AM, David Ahern wrote: > On 7/11/13 7:12 AM, Adrian Hunter wrote: >> The event stream is not always parsable because the format of a sample >> is dependent on the sample_type of the selected event. When there >> is more than one selected event and the sample_types are not the >> same then parsing becomes problematic. A sample can be matched to its >> selected event using the ID that is allocated when the event is opened. >> Unfortunately, to get the ID from the sample means first parsing it. > > Here's an alternative suggestion -- one that does not involve changing > the kernel API or requiring a common denominator in sample_type options. > > perf handles event streams through an mmap which can be directly tied to > an evsel (a single event) when the mmap is created. ie., when events are > read we know exactly which evsel they correspond to. (See > perf_evlist__mmap_per_cpu and perf_evlist__mmap_per_thread and add > struct perf_evsel *evsel entry to struct perf_mmap).
Read that code a bit too quickly. All events for a thread are dropped into the same buffer so as currently organized there is not an easy correlation.
David
> > Commands like perf-record can inject a user event into the stream and > hence the data file every time the evsel changes while walking all of > the mmap's reading events -- very similar to the way finished round is > done. The event would only contain a perf_event_header which is 8 bytes > so this does not add a lot to a data file. As an optimization the evsel > event could only be injected if the sample_types differ. > > Live commands would just use the evsel connected to the mmap -- no > lookups needed which would simplify things a bit processing the events. > > In short, the information to associate event streams to an event (evsel) > is currently available -- it's just being discarded in the many layers. > > I'll try to whip up some code that implements this in the next few days. > > David >
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