Messages in this thread | | | Date | Wed, 19 Jun 2013 17:28:44 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 0/3] tracing/perf: perf_trace_buf/perf_xxx hacks. |
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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.
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