Re: [PATCH] memcg: add tracing for memcg stat updates

Shakeel Butt
2024-10-15T18:39:57Z

On Tue, Oct 15, 2024 at 01:07:30AM GMT, Yosry Ahmed wrote:
> On Wed, Oct 9, 2024 at 5:36 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > The memcg stats are maintained in rstat infrastructure which provides
> > very fast updates side and reasonable read side. However memcg added
> > plethora of stats and made the read side, which is cgroup rstat flush,
> > very slow. To solve that, threshold was added in the memcg stats read
> > side i.e. no need to flush the stats if updates are within the
> > threshold.
> >
> > This threshold based improvement worked for sometime but more stats were
> > added to memcg and also the read codepath was getting triggered in the
> > performance sensitive paths which made threshold based ratelimiting
> > ineffective. We need more visibility into the hot and cold stats i.e.
> > stats with a lot of updates. Let's add trace to get that visibility.
> >
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
>
> One question below, otherwise:
>
> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
>
> > ---
> > include/trace/events/memcg.h | 59 ++++++++++++++++++++++++++++++++++++
> > mm/memcontrol.c | 13 ++++++--
> > 2 files changed, 70 insertions(+), 2 deletions(-)
> > create mode 100644 include/trace/events/memcg.h
> >
> > diff --git a/include/trace/events/memcg.h b/include/trace/events/memcg.h
> > new file mode 100644
> > index 000000000000..913db9aba580
> > --- /dev/null
> > +++ b/include/trace/events/memcg.h
> > @@ -0,0 +1,59 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#undef TRACE_SYSTEM
> > +#define TRACE_SYSTEM memcg
> > +
> > +#if !defined(_TRACE_MEMCG_H) || defined(TRACE_HEADER_MULTI_READ)
> > +#define _TRACE_MEMCG_H
> > +
> > +#include <linux/memcontrol.h>
> > +#include <linux/tracepoint.h>
> > +
> > +
> > +DECLARE_EVENT_CLASS(memcg_rstat,
> > +
> > + TP_PROTO(struct mem_cgroup *memcg, int item, int val),
> > +
> > + TP_ARGS(memcg, item, val),
> > +
> > + TP_STRUCT__entry(
> > + __field(u64, id)
> > + __field(int, item)
> > + __field(int, val)
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->id = cgroup_id(memcg->css.cgroup);
> > + __entry->item = item;
> > + __entry->val = val;
> > + ),
> > +
> > + TP_printk("memcg_id=%llu item=%d val=%d",
> > + __entry->id, __entry->item, __entry->val)
> > +);
> > +
> > +DEFINE_EVENT(memcg_rstat, mod_memcg_state,
> > +
> > + TP_PROTO(struct mem_cgroup *memcg, int item, int val),
> > +
> > + TP_ARGS(memcg, item, val)
> > +);
> > +
> > +DEFINE_EVENT(memcg_rstat, mod_memcg_lruvec_state,
> > +
> > + TP_PROTO(struct mem_cgroup *memcg, int item, int val),
> > +
> > + TP_ARGS(memcg, item, val)
> > +);
> > +
> > +DEFINE_EVENT(memcg_rstat, count_memcg_events,
> > +
> > + TP_PROTO(struct mem_cgroup *memcg, int item, int val),
> > +
> > + TP_ARGS(memcg, item, val)
> > +);
> > +
> > +
> > +#endif /* _TRACE_MEMCG_H */
> > +
> > +/* This part must be outside protection */
> > +#include <trace/define_trace.h>
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c098fd7f5c5e..17af08367c68 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -71,6 +71,10 @@
> >
> > #include <linux/uaccess.h>
> >
> > +#define CREATE_TRACE_POINTS
> > +#include <trace/events/memcg.h>
> > +#undef CREATE_TRACE_POINTS
> > +
> > #include <trace/events/vmscan.h>
> >
> > struct cgroup_subsys memory_cgrp_subsys __read_mostly;
> > @@ -682,7 +686,9 @@ void __mod_memcg_state(struct mem_cgroup *memcg, enum memcg_stat_item idx,
> > return;
> >
> > __this_cpu_add(memcg->vmstats_percpu->state[i], val);
> > - memcg_rstat_updated(memcg, memcg_state_val_in_pages(idx, val));
> > + val = memcg_state_val_in_pages(idx, val);
> > + memcg_rstat_updated(memcg, val);
> > + trace_mod_memcg_state(memcg, idx, val);
> > }
> >
> > /* idx can be of type enum memcg_stat_item or node_stat_item. */
> > @@ -741,7 +747,9 @@ static void __mod_memcg_lruvec_state(struct lruvec *lruvec,
> > /* Update lruvec */
> > __this_cpu_add(pn->lruvec_stats_percpu->state[i], val);
> >
> > - memcg_rstat_updated(memcg, memcg_state_val_in_pages(idx, val));
> > + val = memcg_state_val_in_pages(idx, val);
> > + memcg_rstat_updated(memcg, val);
> > + trace_mod_memcg_lruvec_state(memcg, idx, val);
> > memcg_stats_unlock();
> > }
> >
> > @@ -832,6 +840,7 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
> > memcg_stats_lock();
> > __this_cpu_add(memcg->vmstats_percpu->events[i], count);
> > memcg_rstat_updated(memcg, count);
> > + trace_count_memcg_events(memcg, idx, count);
>
> count here is an unsigned long, and we are casting it to int, right?
>
> Would it be slightly better if the tracepoint uses a long instead of
> int? It's still not ideal but probably better than int.
>

Do you mean something line the following? If this looks good to you then
we can ask Andrew to squash this in the patch.

diff --git a/include/trace/events/memcg.h b/include/trace/events/memcg.h
index 913db9aba580..37812900acce 100644
--- a/include/trace/events/memcg.h
+++ b/include/trace/events/memcg.h
@@ -11,14 +11,14 @@

DECLARE_EVENT_CLASS(memcg_rstat,

- TP_PROTO(struct mem_cgroup *memcg, int item, int val),
+ TP_PROTO(struct mem_cgroup *memcg, int item, long val),

TP_ARGS(memcg, item, val),

TP_STRUCT__entry(
__field(u64, id)
__field(int, item)
- __field(int, val)
+ __field(long, val)
),

TP_fast_assign(
@@ -33,21 +33,21 @@ DECLARE_EVENT_CLASS(memcg_rstat,

DEFINE_EVENT(memcg_rstat, mod_memcg_state,

- TP_PROTO(struct mem_cgroup *memcg, int item, int val),
+ TP_PROTO(struct mem_cgroup *memcg, int item, long val),

TP_ARGS(memcg, item, val)
);

DEFINE_EVENT(memcg_rstat, mod_memcg_lruvec_state,

- TP_PROTO(struct mem_cgroup *memcg, int item, int val),
+ TP_PROTO(struct mem_cgroup *memcg, int item, long val),

TP_ARGS(memcg, item, val)
);

DEFINE_EVENT(memcg_rstat, count_memcg_events,

- TP_PROTO(struct mem_cgroup *memcg, int item, int val),
+ TP_PROTO(struct mem_cgroup *memcg, int item, long val),

TP_ARGS(memcg, item, val)
);