Re: [RFC] perf_events: support for uncore a.k.a. nest units

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-04-20 at 19:55 +0800, Lin Ming wrote:

> > One thing not on that list, which should happen first I guess, is to
> > remove hw_perf_group_sched_in(). The idea is to add some sort of
> > transactional API to the struct pmu, so that we can delay the
> > schedulability check until commit time (and roll back when it fails).
> > 
> > Something as simple as:
> > 
> >   struct pmu {
> >     void start_txn(struct pmu *);
> >     void commit_txn(struct pmu *);
> > 
> >     ,,,
> >   };
> 
> Could you please explain a bit more?
> 
> Does it mean that "start_txn" perform the schedule events stuff
> and "commit_txn" perform the assign events stuff?
> 
> Does "commit time" mean the actual activation in hw_perf_enable?

No, the idea behind hw_perf_group_sched_in() is to not perform
schedulability tests on each event in the group, but to add the group as
a whole and then perform one test.

Of course, when that test fails, you'll have to roll-back the whole
group again.

So start_txn (or a better name) would simply toggle a flag in the pmu
implementation that will make pmu::enable() not perform the
schedulablilty test.

Then commit_txn() will perform the schedulability test (so note the
method has to have a !void return value, my mistake in the earlier
email).

This will allow us to use the regular
kernel/perf_event.c::group_sched_in() and all the rollback code.
Currently each hw_perf_group_sched_in() implementation duplicates all
the rolllback code (with various bugs).



We must get rid of all weak hw_perf_*() functions before we can properly
consider multiple struct pmu implementations.