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

[Ravi Jonnalagadda]

On Sun, Oct 4, 2026 at 2:16 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> 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. */
> > +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)
>
> Can you move the nid after the two unsigned long values below. This
> creates a structure, and on 64bit machines, this adds a 4 byte padding
> in the middle of the ring buffer.

Hi Steve,

Thank you.  Will do; in the next version the 8-byte fields come first
and nid follows them.

>
> > +             __field(unsigned long, target_value)
> > +             __field(unsigned long, current_value)
> > +     ),
> > +
> > +     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()) {
>
> If you move the trace_damos_node_eligible_mem_bp_enabled() before the
> goal->metric check, you will save yourself a branch when tracing is
> off. As the trace_*_enabled() is a static branch (nop or jmp) it has no
> cmp branch, but the above will keep the goal->metric still exposed. By
> swapping the two compares, the goal->metric check will be hidder behind
> the 'nop' when tracing is disabled.
>
> -- Steve

Makes sense; I will put the trace_*_enabled() check first.  SJ asked for
the event to cover every quota goal rather than this metric only, so
the next version may not need the metric check at all, but the enabled
check will lead either way.

Thanks,
Ravi

>
>
> > +                     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
>