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

[Yosry Ahmed]

On Tue, Oct 15, 2024 at 11:39 AM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx> 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@xxxxxxxxx>
> >
> > One question below, otherwise:
> >
> > Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> >
> > > ---
> > >  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.

Yes, unless you have a better way to also accommodate the unsigned
long value in __count_memcg_events().

>
>
> 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)
>  );
>