Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get the selected hardware PMU conuter

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 17, 2015 at 07:29:07PM +0800, Wangnan (F) wrote:
> 
> 
> On 2015/7/17 19:05, Peter Zijlstra wrote:
> >On Fri, Jul 17, 2015 at 06:43:35PM +0800, kaixu xia wrote:
> >>The function bpf_read_pmu() can get the specific map key, convert
> >>the corresponding map value to the pointer to struct perf_event and
> >>return the Hardware PMU counter value.
> >Thanks for having me on Cc :/
> >
> >>Signed-off-by: kaixu xia <xiakaixu@huawei.com>
> >>---
> >>+static u64 bpf_read_pmu(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
> >>+{
> >>+	void *value = (void *) (unsigned long) r1;
> >>+	struct perf_event *event;
> >>+	u64 count;
> >>+
> >>+	if (!value || !(*(unsigned long *)value))
> >>+		return 0;
> >>+
> >>+	event = (struct perf_event *)(*(unsigned long *)value);
> >>+
> >>+	if (event->state == PERF_EVENT_STATE_ACTIVE)
> >>+		event->pmu->read(event);
> >>+
> >>+	count = local64_read(&event->count);
> >>+
> >>+	return count;
> >>+}
> >Hell no, that's way broken.
> What about calling perf_event_read_value() then?

Depends on what all you need, if you need full perf events to work then
yes perf_event_read_value() is your only option.

But note that that requires scheduling, so you cannot actually use it
for tracing purposes etc..