On Mon, 11 Sep 2023 16:31:27 -0400 Steven Rostedt <rost...@goodmis.org> wrote:
> On Mon, 11 Sep 2023 19:05:04 +0000
> SeongJae Park <s...@kernel.org> wrote:
>
> > > Also, this if statement is only done when the trace event is enabled, so
> > > it's equivalent to:
> > >
> > > if (trace_damos_before_apply_enabled()) {
> > > if (sdx >= 0)
> > > trace_damos_before_apply(cidx, sidx, tidx, r,
> > > damon_nr_regions(t));
> > > }
> >
> > Again, thank you very much for letting me know this awesome feature.
> > However,
> > sidx is supposed to be always >=0 here, since kdamond is running in single
> > thread and hence no race is expected. If it exists, it's a bug. So, I
> > wouldn't make this change. Appreciate again for letting me know this very
> > useful feature, and please let me know if I'm missing something, though!
>
> The race isn't with your code, but the enabling of tracing.
>
> Let's say you enable tracing just ass it passed the first:
>
> if (trace_damos_before_apply_enabled()) {
>
> damon_for_each_scheme(siter, c) {
> if (siter == s)
> break;
> sidx++;
> }
> damon_for_each_target(titer, c) {
> if (titer == t)
> break;
> tidx++;
> }
>
> Now, sidx and tidx is zero (when they were not computed, thus, they
> shouldn't be zero.
>
> Then tracing is fully enabled here, and now we enter:
>
> if (trace_damos_before_apply_enabled()) {
> trace_damos_before_apply(cidx, sidx, tidx, r,
> damon_nr_regions(t));
> }
>
> Now the trace event is hit with sidx and tidx zero when they should not be.
> This could confuse you when looking at the report.
Thank you so much for enlightening me with this kind explanation, Steve! And
this all make sense. I will follow your suggestion in the next spin.
>
> What I suggested was to initialize sidx to zero,
Nit. Initialize to not zero but -1, right?
> set it in the first trace_*_enabled() check, and ignore calling the
> tracepoint if it's not >= 0.
>
> -- Steve
>
Thanks,
SJ
