[PATCH 0/3] Teach perf_trace_##call() to check hlist_empty(perf_events)

[Oleg Nesterov <oleg@redhat.com>]

Sorry for double post, forgot to cc lkml...

On 07/19, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > Hello.
> >
> > The patches are the same, I only tried to update the changelogs a bit.
> > I am also quoting my old email below, to explain what this hack tries
> > 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.
>
> Nice improvement.
>
> Peter, Steve, any objections?

Ingo,

It seems that everybody agree with this hack but it was forgotten,
let me resend it again.

The only change is that I added the following tags:

	Tested-by: David Ahern <dsahern@gmail.com>
	Reviewed-and-Acked-by: Steven Rostedt <rostedt@goodmis.org>

Oleg.

 include/trace/events/sched.h |   22 ++++++++--------------
 include/trace/ftrace.h       |   33 ++++++++++++++++++++-------------
 2 files changed, 28 insertions(+), 27 deletions(-)