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

From: Oleg Nesterov
Date: Wed Jun 19 2013 - 11:33:58 EST

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

if (hlist_empty(event_call->perf_events))

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))

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.


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