Re: [PATCH] mm/damon: add damos_node_eligible_mem_bp tracepoint

[Kunwu Chan <kunwu.chan@gmail.com>]

Hi Ravi,

I agree with SJ that this would be more useful as a general quota-goal
tracepoint rather than being specific to NODE_ELIGIBLE_MEM_BP. The
target_value and current_value fields are not metric-specific.

On Sat,  3 Oct 2026 13:27:27 -0700 Ravi Jonnalagadda <ravis.opensrc@gmail.com> wrote:

> Add a tracepoint for DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP quota goal
> evaluation.  The tracepoint fires once per quota-score computation when
> the goal metric is DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP, exposing
> context_idx, scheme_idx, nid, target_value, and current_value.
> current_value is reported as the quota tuning sees it, after the goal's
> complement setting is applied.
> 
> The DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP metric expresses node memory
> occupancy as basis points of eligible bytes.  This information is not
> exposed through any existing external interface (perf stat, /proc, or
> sysfs), so the tracepoint is the only way for userspace to observe the
> target and current eligible-memory basis point values at quota-scoring
> time.
> 
> Link: https://lore.kernel.org/linux-mm/20260428030520.701-1-ravis.opensrc@gmail.com/
> Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
> ---
>  include/trace/events/damon.h | 31 +++++++++++++++++++++++++++++++
>  mm/damon/core.c              | 16 ++++++++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/include/trace/events/damon.h b/include/trace/events/damon.h
> index 8851727ae162..2c7249cc7c88 100644
> --- a/include/trace/events/damon.h
> +++ b/include/trace/events/damon.h
> @@ -74,6 +74,37 @@ TRACE_EVENT(damos_esz,
>  			__entry->esz)
>  );
>  
> +/* Per-tick DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP goal evaluation. */

The comment says "Per-tick", but the event is emitted for each matching
goal in damos_quota_score(). "Per quota-goal evaluation" would be more
precise.

> +TRACE_EVENT(damos_node_eligible_mem_bp,
> +
> +	TP_PROTO(unsigned int context_idx, unsigned int scheme_idx,
> +		int nid,
> +		unsigned long target_value, unsigned long current_value),
> +
> +	TP_ARGS(context_idx, scheme_idx, nid, target_value, current_value),
> +
> +	TP_STRUCT__entry(
> +		__field(unsigned int, context_idx)
> +		__field(unsigned int, scheme_idx)
> +		__field(int, nid)
> +		__field(unsigned long, target_value)
> +		__field(unsigned long, current_value)

Also, context_idx is currently always 0 here, since
damos_quota_score() only has struct damon_ctx * and no context index.
If this tracepoint is generalized, it may be worth considering how the
context/scheme are identified rather than introducing another tracepoint 
field that is currently always 0.

Thanks,
Kunwu

> +	),
> +
> +	TP_fast_assign(
> +		__entry->context_idx = context_idx;
> +		__entry->scheme_idx = scheme_idx;
> +		__entry->nid = nid;
> +		__entry->target_value = target_value;
> +		__entry->current_value = current_value;
> +	),
> +
> +	TP_printk("ctx_idx=%u scheme_idx=%u nid=%d target_value=%lu current_value=%lu",
> +			__entry->context_idx, __entry->scheme_idx,
> +			__entry->nid,
> +			__entry->target_value, __entry->current_value)
> +);
> +
>  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 b63e60ef8990..6ddd15dc4f55 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3294,6 +3294,22 @@ static unsigned long damos_quota_score(struct damon_ctx *c, struct damos *s)
>  		highest_score = max(highest_score,
>  				mult_frac(goal->current_value, 10000,
>  					goal->target_value));
> +
> +		if (goal->metric == DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP &&
> +		    trace_damos_node_eligible_mem_bp_enabled()) {
> +			unsigned int cidx = 0, sidx = 0;
> +			struct damos *siter;
> +
> +			damon_for_each_scheme(siter, c) {
> +				if (siter == s)
> +					break;
> +				sidx++;
> +			}
> +			trace_damos_node_eligible_mem_bp(cidx, sidx,
> +					goal->nid,
> +					goal->target_value,
> +					goal->current_value);
> +		}
>  	}
>  
>  	return highest_score;
> 
> base-commit: 763ad0211c7b587344f03bc4d1299810aeb736f4
> -- 
> 2.54.0 (Apple Git-157)
> 
> 

Sent using hkml (https://github.com/sjp38/hackermail)