Re: [RFC][PATCH] perf: Implement read_group() PMU operation

[Peter Zijlstra <peterz@infradead.org>]

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.