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

[Ravi Jonnalagadda]

On Mon, Oct 5, 2026 at 3:04 AM Kunwu Chan <kunwu.chan@xxxxxxxxx> wrote:
>
>
> 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.

Hi Kunwu,

Thank you for the review.  Agreed; I will make it a general quota goal
tracepoint, as I wrote in my reply to SJ.

>
> On Sat,  3 Oct 2026 13:27:27 -0700 Ravi Jonnalagadda <ravis.opensrc@xxxxxxxxx> 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@xxxxxxxxx/
> > Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@xxxxxxxxx>
> > ---
> >  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.

Right, I will fix the comment.

>
> > +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.

I kept context_idx for consistency with damos_esz and
damos_before_apply, which carry the same field and also pass 0 today,
since a kdamond runs one context.  I will follow whatever SJ prefers
for identifying the context in the generalized event.

Thanks,
Ravi.

>
> 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)
> >
> >
>
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