On Tue, Jul 21, 2015 at 06:50:45PM -0700, Sukadev Bhattiprolu wrote:

We are trying to use the following interface:

	start_txn(pmu, PERF_PMU_TXN_READ);

	perf_event_read(leader);
	list_for_each(sibling, &leader->sibling_list, group_entry)
		perf_event_read(sibling)

	pmu->commit_txn(pmu);

with the idea that the PMU driver would save the type of transaction in
->start_txn() and use in ->read() and ->commit_txn().

But since ->start_txn() and the ->read() operations could happen on different
CPUs (perf_event_read() uses the event->oncpu to schedule a call), the PMU
driver cannot use a per-cpu variable to save the state in ->start_txn().

or is there better way?

I've not woken up yet, and not actually fully read the email, but can
you stuff the entire above chunk inside the IPI?

I think you could then actually optimize __perf_event_read() as well,
because all these events should be on the same context, so no point in
calling update_*time*() for every event or so.