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

Yosry Ahmed
2024-10-15T08:03:32Z

> > > > > > > @@ -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
> > >
> > > Does reading side mean the one reading /sys/kernel/tracing/trace will do
> > > the translation from enums to string?
> > >
> > > > > 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?
> > >
> > > Cc Daniel & Martin
> > >
> > > I was planning to use bpftrace which can use dwarf/btf to convert the
> > > raw int to its enum string. Martin provided the following command to
> > > extract the translation from the kernel.
> > >
> > > $ bpftool btf dump file /sys/kernel/btf/vmlinux | grep -A10 node_stat_item
> > > [2264] ENUM 'node_stat_item' encoding=UNSIGNED size=4 vlen=46
> > > 'NR_LRU_BASE' val=0
> > > 'NR_INACTIVE_ANON' val=0
> > > 'NR_ACTIVE_ANON' val=1
> > > 'NR_INACTIVE_FILE' val=2
> > > 'NR_ACTIVE_FILE' val=3
> > > 'NR_UNEVICTABLE' val=4
> > > 'NR_SLAB_RECLAIMABLE_B' val=5
> > > 'NR_SLAB_UNRECLAIMABLE_B' val=6
> > > 'NR_ISOLATED_ANON' val=7
> > > 'NR_ISOLATED_FILE' val=8
> > > ...
> > >
> > > My point is userspace tools can use existing infra to extract this
> > > information.
> > >
> > > However I am not against adding __print_symbolic() (but without any
> > > duplication), so users reading /sys/kernel/tracing/trace directly can
> > > see more useful information as well. Please post a follow up patch after
> > > this one.
> >
> > I briefly looked into this and I think it would be annoying to have
> > this, unfortunately. Even if we rework the existing arrays with memcg
> > stat names to be in a format conforming to tracing, we would need to
> > move them out to a separate header to avoid a circular dependency.
> >
> > Additionally, for __count_memcg_events() things will be more
> > complicated because the names are not in an array in memcontrol.c, but
> > we use vm_event_name() and the relevant names are part of a larger
> > array, vmstat_text, which we would need to rework similarly.
> >
> > I think this would be easier to implement if we can somehow provide a
> > callback that returns the name based on the index, rather than an
> > array. But even then, we would need to specify a different callback
> > for each event, so it won't be as simple as specifying the callback in
> > the event class.
> >
> > All in all, unless we realize there is something that is fundamentally
> > more difficult to do in userspace, I think it's not worth adding this
> > unfortunately :/
>
> Turned out to be quite straightforward to do in userspace:
> https://github.com/bpftrace/bpftrace/pull/3515 .
>
> A nice property is the resolution occurs out of line and saves the
> kernel some cycles in the fast path.

This is really neat, thanks for doing this! Native support in bpftrace
is definitely better than manually correlating the values, especially
with large enums with 10s of values like the ones we are talking about
here.