Re: [PATCH 1/2] ftrace: record command lines at more appropriate moment

Ian Munsie
2010-07-28T11:44:28Z

Excerpts from Li Zefan's message of Wed Jul 28 12:55:54 +1000 2010:
> > @@ -1112,6 +1110,7 @@ tracing_generic_entry_update(struct trace_entry *entry, unsigned long flags,
> > {
> > struct task_struct *tsk = current;
> >
> > + tracing_record_cmdline(tsk);
>
> Now this function is called everytime a tracepoint is triggered, so
> did you run some benchmarks to see if the performance is improved
> or even worse?

Admittedly when I posted the patch I had not done that. For the below
benchmark I isolated the trace_sched_switch tracepoint from the
context_switch function (since it is called often) into it's own function
(tp_benchmark) which I could then run the ftrace function profiler on
while the tracepoint was active through debugfs.

On my test system there is a performance hit for an active event of
~0.233 us per event (which I have now reduced to ~0.127 us by inlining
tracing_record_cmdline and trace_save_cmdline). At least that is only on
active events as opposed to every single context switch as before.

Before:
Function Hit Time Avg s^2
-------- --- ---- --- ---
.tp_benchmark 1494 2699.670 us 1.807 us 0.536 us
.tp_benchmark 212 357.546 us 1.686 us 0.363 us
.tp_benchmark 215 389.984 us 1.813 us 0.404 us
.tp_benchmark 649 1116.156 us 1.719 us 0.626 us
.tp_benchmark 273 483.530 us 1.771 us 0.350 us
.tp_benchmark 333 599.600 us 1.800 us 0.378 us
.tp_benchmark 203 355.038 us 1.748 us 0.351 us
.tp_benchmark 270 473.222 us 1.752 us 0.360 us

After existing patch:
Function Hit Time Avg s^2
-------- --- ---- --- ---
.tp_benchmark 1427 2815.906 us 1.973 us 0.623 us
.tp_benchmark 358 645.550 us 1.803 us 0.240 us
.tp_benchmark 437 867.762 us 1.985 us 0.684 us
.tp_benchmark 701 1445.618 us 2.062 us 0.906 us
.tp_benchmark 121 257.166 us 2.125 us 0.949 us
.tp_benchmark 162 329.536 us 2.034 us 0.671 us
.tp_benchmark 216 448.420 us 2.076 us 0.754 us
.tp_benchmark 238 452.244 us 1.900 us 0.384 us

With inlining:
Function Hit Time Avg s^2
-------- --- ---- --- ---
.tp_benchmark 1478 2834.292 us 1.917 us 0.451 us
.tp_benchmark 316 583.166 us 1.845 us 0.227 us
.tp_benchmark 160 312.752 us 1.954 us 0.302 us
.tp_benchmark 687 1251.652 us 1.821 us 0.445 us
.tp_benchmark 177 352.310 us 1.990 us 0.451 us
.tp_benchmark 324 603.848 us 1.863 us 0.239 us
.tp_benchmark 150 284.444 us 1.896 us 0.343 us
.tp_benchmark 339 617.716 us 1.822 us 0.215 us

> Another problem in this patch is, tracing_generic_entry_update() is also
> called by perf, but cmdline recoding is not needed in perf.

That's a good point - I could move the call into
trace_buffer_lock_reserve so that perf does not get the unneeded
overhead. Actually there's probably no reason I couldn't put it in
__trace_buffer_unlock_commit to avoid the overhead if the event has been
filtered out.

Anyway, what do you think? Is the extra overhead per event acceptable?
I'll go ahead and respin the patch to remove the overhead in the perf
case for the moment.

Cheers,
-Ian