linux-kernel - Re: [PATCH 1/2] mm/damon/core: add a tracepoint for damos apply target regions

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Message-Id: <20230911190504.102317-1-sj@kernel.org>
Date:   Mon, 11 Sep 2023 19:05:04 +0000
From:   SeongJae Park <sj@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     SeongJae Park <sj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        damon@...ts.linux.dev, linux-mm@...ck.org,
        linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/damon/core: add a tracepoint for damos apply target regions

Hi Steven,

On Mon, 11 Sep 2023 14:19:55 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 11 Sep 2023 04:59:07 +0000
> SeongJae Park <sj@...nel.org> wrote:
> 
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -950,6 +950,28 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
> >  	struct timespec64 begin, end;
> >  	unsigned long sz_applied = 0;
> >  	int err = 0;
> > +	/*
> > +	 * We plan to support multiple context per kdamond, as DAMON sysfs
> > +	 * implies with 'nr_contexts' file.  Nevertheless, only single context
> > +	 * per kdamond is supported for now.  So, we can simply use '0' context
> > +	 * index here.
> > +	 */
> > +	unsigned int cidx = 0;
> > +	struct damos *siter;		/* schemes iterator */
> > +	unsigned int sidx = 0;
> > +	struct damon_target *titer;	/* targets iterator */
> > +	unsigned int tidx = 0;
> > +
> 
> If this loop is only for passing sidx and tidx to the trace point,

You're correct.

> you can add around it:
> 
> 	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++;
> > +	}
> 
> 	}
> 
> 
> And then this loop will only be done if that trace event is enabled.

Today I learned yet another great feature of the tracing framework.  Thank you
Steven, I will add that to the next spin of this patchset!

> 
> To prevent races, you may also want to add a third parameter, or initialize
> them to -1:
> 
> 	sidx = -1;
> 
> 	if (trace_damo_before_apply_enabled()) {
> 		sidx = 0;
> 		[..]
> 	}
> 
> And you can change the TRACE_EVENT() TO TRACE_EVENT_CONDITION():
> 
> TRACE_EVENT_CONDITION(damos_before_apply,
> 
> 	TP_PROTO(...),
> 
> 	TP_ARGS(...),
> 
> 	TP_CONDITION(sidx >= 0),
> 
> and the trace event will not be called if sidx is less than zero.
> 
> 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!


Thanks,
SJ

> 
> -- Steve
> 
> 
> 
> >  
> >  	if (c->ops.apply_scheme) {
> >  		if (quota->esz && quota->charged_sz + sz > quota->esz) {
> > @@ -964,8 +986,11 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
> >  		ktime_get_coarse_ts64(&begin);
> >  		if (c->callback.before_damos_apply)
> >  			err = c->callback.before_damos_apply(c, t, r, s);
> > -		if (!err)
> > +		if (!err) {
> > +			trace_damos_before_apply(cidx, sidx, tidx, r,
> > +					damon_nr_regions(t));
> >  			sz_applied = c->ops.apply_scheme(c, t, r, s);
> > +		}
> >  		ktime_get_coarse_ts64(&end);
> >  		quota->total_charged_ns += timespec64_to_ns(&end) -
> >  			timespec64_to_ns(&begin);
> > -- 
>