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From:   SeongJae Park <sj@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     SeongJae Park <sj@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        damon@lists.linux.dev, linux-mm@kvack.org,
        linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm/damon/core: add a tracepoint for damos apply target regions
Date:   Tue, 12 Sep 2023 01:43:08 +0000
Message-Id: <20230912014308.38298-1-sj@kernel.org>
In-Reply-To: <20230911165144.5002a0ac@gandalf.local.home>
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On Mon, 11 Sep 2023 16:51:44 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 11 Sep 2023 20:36:42 +0000
> SeongJae Park <sj@kernel.org> wrote:
> 
> > > 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?
> 
> Yeah, but I was also thinking of the reset of it too :-p
> 
> 	sidx = -1;
> 
> 	if (trace_damos_before_apply_enabled()) {
> 		sidx = 0;

Thank you for clarifying, Steve :)

Nevertheless, since the variable is unsigned int, I would need to use UINT_MAX
instead.  To make the code easier to understand, I'd prefer to add a third
parameter, as you suggested as another option at the original reply, like
below:

--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -997,6 +997,7 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
        unsigned int sidx = 0;
        struct damon_target *titer;     /* targets iterator */
        unsigned int tidx = 0;
+       bool do_trace = false;

        /* get indices for trace_damos_before_apply() */
        if (trace_damos_before_apply_enabled()) {
@@ -1010,6 +1011,7 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
                                break;
                        tidx++;
                }
+               do_trace = true;
        }

        if (c->ops.apply_scheme) {
@@ -1036,7 +1038,7 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
                        err = c->callback.before_damos_apply(c, t, r, s);
                if (!err) {
                        trace_damos_before_apply(cidx, sidx, tidx, r,
-                                       damon_nr_regions(t));
+                                       damon_nr_regions(t), do_trace);
                        sz_applied = c->ops.apply_scheme(c, t, r, s);
                }
                ktime_get_coarse_ts64(&end);


Thanks,
SJ

> 
> -- Steve
> 
> 
> > 
> > > set it in the first trace_*_enabled() check, and ignore calling the
> > > tracepoint if it's not >= 0.
> > >