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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: Mon, 11 Sep 2023 20:36:42 +0000
Message-Id: <20230911203642.1788-1-sj@kernel.org>
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On Mon, 11 Sep 2023 16:31:27 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 11 Sep 2023 19:05:04 +0000
> SeongJae Park <sj@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