Re: [PATCH 0/3] tracing/perf: perf_trace_buf/perf_xxx hacks.

[Oleg Nesterov <oleg@redhat.com>]

On 06/19, Peter Zijlstra wrote:
>
> I'm probably missing something obviuos, but what are we trying to do?

Say, "perf record -e sched:sched_switch -p1".

Every task except /sbin/init will do perf_trace_sched_switch() and
perf_trace_buf_prepare() + perf_trace_buf_submit for no reason(),
it doesn't have a counter.

So it makes sense to add the fast-path check at the start of
perf_trace_##call(),

	if (hlist_empty(event_call->perf_events))
		return;

The problem is, we should not do this if __task != NULL (iow, if
DECLARE_EVENT_CLASS() uses __perf_task()), perf_tp_event() has the
additional code for this case.

So we should do

	if (!__task && hlist_empty(event_call->perf_events))
		return;

But __task is changed by "{ assign; }" block right before
perf_trace_buf_submit(). Too late for the fast-path check,
we already called perf_trace_buf_prepare/fetch_regs.

So. After 2/3 __perf_task() (and __perf_count/addr) is called
when ftrace_get_offsets_##call(args) evaluates the arguments,
and we can check !__task && hlist_empty() right after that.

Oleg.