Messages in this thread | | | Date | Fri, 19 Jul 2013 09:43:39 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH RESEND 0/3] Teach perf_trace_##call() to check hlist_empty(perf_events) |
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* 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?
Thanks,
Ingo
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