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

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Oct 9, 2024 at 6:08 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 9 Oct 2024 17:46:22 -0700
> Yosry Ahmed <yosryahmed@google.com> wrote:
>
> > > +++ 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);
> >
> > Is it too unreasonable to include the stat name?
> >
> > The index has to be correlated with the kernel config and perhaps even
> > version. It's not a big deal, but if performance is not a concern when
> > tracing is enabled anyway, maybe we can lookup the name here (or in
> > TP_fast_assign()).
>
> What name? Is it looked up from idx? If so, you can do it on the reading of
> the trace event where performance is not an issue. See the __print_symbolic()
> and friends in samples/trace_events/trace-events-sample.h

Yeah they can be found using idx. Thanks for referring us to
__print_symbolic(), I suppose for this to work we need to construct an
array of {idx, name}. I think we can replace the existing memory_stats
and memcg1_stats/memcg1_stat_names arrays with something that we can
reuse for tracing, so we wouldn't need to consume extra space.

Shakeel, what do you think?