linux-kernel - Re: [PATCH 2/2] mm/damon: add trace event for effective size quota

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Message-ID: <20250709122123.779c874f@batman.local.home>
Date: Wed, 9 Jul 2025 12:21:23 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 damon@...ts.linux.dev, kernel-team@...a.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/damon: add trace event for effective size quota

On Fri,  4 Jul 2025 15:14:08 -0700
SeongJae Park <sj@...nel.org> wrote:

> Aim-oriented DAMOS quota auto-tuning is an important and recommended
> feature for DAMOS users.  Add a trace event for the observability of the
> tuned quota and tuning itself.
> 
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
>  include/trace/events/damon.h | 26 ++++++++++++++++++++++++++
>  mm/damon/core.c              | 20 +++++++++++++++++++-
>  2 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/include/trace/events/damon.h b/include/trace/events/damon.h
> index 32c611076023..36b2cdf47dce 100644
> --- a/include/trace/events/damon.h
> +++ b/include/trace/events/damon.h
> @@ -9,6 +9,32 @@
>  #include <linux/types.h>
>  #include <linux/tracepoint.h>
>  
> +TRACE_EVENT_CONDITION(damos_esz,
> +
> +	TP_PROTO(unsigned int context_idx, unsigned int scheme_idx,
> +		unsigned long esz, bool do_trace),
> +
> +	TP_ARGS(context_idx, scheme_idx, esz, do_trace),
> +
> +	TP_CONDITION(do_trace),

Please explain to me why you are using a conditional here?

> +
> +	TP_STRUCT__entry(
> +		__field(unsigned int, context_idx)
> +		__field(unsigned int, scheme_idx)
> +		__field(unsigned long, esz)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->context_idx = context_idx;
> +		__entry->scheme_idx = scheme_idx;
> +		__entry->esz = esz;
> +	),
> +
> +	TP_printk("ctx_idx=%u scheme_idx=%u esz=%lu",
> +			__entry->context_idx, __entry->scheme_idx,
> +			__entry->esz)
> +);
> +
>  TRACE_EVENT_CONDITION(damos_before_apply,
>  
>  	TP_PROTO(unsigned int context_idx, unsigned int scheme_idx,
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 57a1ace4d10d..6019b8ec4bba 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2011,12 +2011,26 @@ static void damos_set_effective_quota(struct damos_quota *quota)
>  	quota->esz = esz;
>  }
>  
> +static void damos_trace_esz(struct damon_ctx *c, struct damos *s,
> +		struct damos_quota *quota)
> +{
> +	unsigned int cidx = 0, sidx;
> +	struct damos *siter;
> +
> +	damon_for_each_scheme(siter, c) {
> +		if (siter == s)
> +			break;
> +		sidx++;
> +	}
> +	trace_damos_esz(cidx, sidx, quota->esz, true);

It's set to true, so it's not even a conditional anymore. The compiler
will likely optimize it out!

-- Steve


> +}
> +
>  static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
>  {
>  	struct damos_quota *quota = &s->quota;
>  	struct damon_target *t;
>  	struct damon_region *r;
> -	unsigned long cumulated_sz;
> +	unsigned long cumulated_sz, cached_esz;
>  	unsigned int score, max_score = 0;
>  
>  	if (!quota->ms && !quota->sz && list_empty(&quota->goals))
> @@ -2030,7 +2044,11 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
>  		quota->total_charged_sz += quota->charged_sz;
>  		quota->charged_from = jiffies;
>  		quota->charged_sz = 0;
> +		if (trace_damos_esz_enabled())
> +			cached_esz = quota->esz;
>  		damos_set_effective_quota(quota);
> +		if (trace_damos_esz_enabled() && quota->esz != cached_esz)
> +			damos_trace_esz(c, s, quota);
>  	}
>  
>  	if (!c->ops.get_scheme_score)