Messages in this thread |  | | Date | Tue, 17 Feb 2015 11:03:01 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH] perf: Implement read_group() PMU operation |
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On Tue, Feb 17, 2015 at 12:33:12AM -0800, Sukadev Bhattiprolu wrote: > Peter Zijlstra [peterz@infradead.org] wrote: > | > --- a/kernel/events/core.c > | > +++ b/kernel/events/core.c > | > @@ -3549,10 +3549,43 @@ static int perf_event_read_group(struct perf_event *event, > | > | You also want perf_output_read_group(). > > Ok. Will look into it. We currently don't support sampling with > the 24x7 counters but we should make sure that the new interface > fits correctly.
One thing someone 'could' do is group them together with a software event that _can_ sample, and then use SAMPLE_READ to periodically stuff values into the buffer.
> | Since ->read() has a void return value, we can delay its effect, so I'm > | currently thinking we might want to extend the transaction interface for > | this; give pmu::start_txn() a flags argument to indicate scheduling > | (add) or reading (read). > | > | So we'd end up with something like: > | > | pmu->start_txn(pmu, PMU_TXN_READ); > | > | leader->read(); > | > | for_each_sibling() > | sibling->read(); > | > | pmu->commit_txn(); > > So, each of the ->read() calls is really "appending a counter" to a > list of counters that the PMU should read and the values for the counters > (results of the read) are only available after the commit_txn() right?
Correct.
> In which case, perf_event_read_group() would then follow this commit_txn() > with its "normal" read, and the PMU would return the result cached during > ->commit_txn(). If so, we need a way to invalidate the cached result ?
I was thinking of breaking up that code into two loops, once to call ->read() and update states, the second to use the now up-to-date data and frob it into the stream.
But I must say I've not entirely given it much thought. But that way you're not stuck with this cache and related problems.
> | after which we can use the values updated by the read calls. The trivial > | no-support implementation lets read do its immediate thing like it does > | now. > | > | A more complex driver can then collect the actual counter values and > | execute one hypercall using its pre-allocated memory. > > the hypercall should happen in the ->commit_txn() right ?
Yah. Of course, if a ->read() is not part of a txn then it must do the hypercall for just the one value.
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