linux-kernel - Re: [PATCH] memcg: introduce kfuncs for fetching memcg stats

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Message-ID: <82c392dc-e750-43d9-9394-1df00a366ae0@gmail.com>
Date: Mon, 6 Oct 2025 13:33:07 -0700
From: JP Kobryn <inwardvessel@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Michal Koutný
 <mkoutny@...e.com>, Yosry Ahmed <yosryahmed@...gle.com>,
 Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 LKML <linux-kernel@...r.kernel.org>,
 "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
 linux-mm <linux-mm@...ck.org>, bpf <bpf@...r.kernel.org>,
 Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: introduce kfuncs for fetching memcg stats

On 10/1/25 3:25 PM, Alexei Starovoitov wrote:
> On Tue, Sep 30, 2025 at 9:57 PM JP Kobryn <inwardvessel@...il.com> wrote:
[..]
>>
>> There is a significant perf benefit when using this approach. In terms of
>> elapsed time, the kfuncs allow a bpf cgroup iterator program to outperform
>> the traditional file reading method, saving almost 80% of the time spent in
>> kernel.
>>
>> control: elapsed time
>> real    0m14.421s
>> user    0m0.183s
>> sys     0m14.184s
>>
>> experiment: elapsed time
>> real    0m3.250s
>> user    0m0.225s
>> sys     0m2.916s
> 
> Nice, but github repo somewhere doesn't guarantee that
> the work is equivalent.
> Please add it as a selftest/bpf instead.
> Like was done in commit
> https://lore.kernel.org/bpf/20200509175921.2477493-1-yhs@fb.com/
> to demonstrate equivalence of 'cat /proc' vs iterator approach.

Sure, I'll relocate the test code there.

[..]
>> ---
>>   mm/memcontrol.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 67 insertions(+)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 8dd7fbed5a94..aa8cbf883d71 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -870,6 +870,73 @@ unsigned long memcg_events_local(struct mem_cgroup *memcg, int event)
>>   }
>>   #endif
>>
>> +static inline struct mem_cgroup *memcg_from_cgroup(struct cgroup *cgrp)
>> +{
>> +       return cgrp ? mem_cgroup_from_css(cgrp->subsys[memory_cgrp_id]) : NULL;
>> +}
>> +
>> +__bpf_kfunc static void memcg_flush_stats(struct cgroup *cgrp)
>> +{
>> +       struct mem_cgroup *memcg = memcg_from_cgroup(cgrp);
>> +
>> +       if (!memcg)
>> +               return;
>> +
>> +       mem_cgroup_flush_stats(memcg);
>> +}
> 
> css_rstat_flush() is sleepable, so this kfunc must be sleepable too.
> Not sure about the rest.

Good catch. I'll add the sleepable flag where it's needed.

> 
>> +
>> +__bpf_kfunc static unsigned long memcg_node_stat_fetch(struct cgroup *cgrp,
>> +               enum node_stat_item item)
>> +{
>> +       struct mem_cgroup *memcg = memcg_from_cgroup(cgrp);
>> +
>> +       if (!memcg)
>> +               return 0;
>> +
>> +       return memcg_page_state_output(memcg, item);
>> +}
>> +
>> +__bpf_kfunc static unsigned long memcg_stat_fetch(struct cgroup *cgrp,
>> +               enum memcg_stat_item item)
>> +{
>> +       struct mem_cgroup *memcg = memcg_from_cgroup(cgrp);
>> +
>> +       if (!memcg)
>> +               return 0;
>> +
>> +       return memcg_page_state_output(memcg, item);
>> +}
>> +
>> +__bpf_kfunc static unsigned long memcg_vm_event_fetch(struct cgroup *cgrp,
>> +               enum vm_event_item item)
>> +{
>> +       struct mem_cgroup *memcg = memcg_from_cgroup(cgrp);
>> +
>> +       if (!memcg)
>> +               return 0;
>> +
>> +       return memcg_events(memcg, item);
>> +}
>> +
>> +BTF_KFUNCS_START(bpf_memcontrol_kfunc_ids)
>> +BTF_ID_FLAGS(func, memcg_flush_stats)
>> +BTF_ID_FLAGS(func, memcg_node_stat_fetch)
>> +BTF_ID_FLAGS(func, memcg_stat_fetch)
>> +BTF_ID_FLAGS(func, memcg_vm_event_fetch)
>> +BTF_KFUNCS_END(bpf_memcontrol_kfunc_ids)
> 
> At least one of them must be sleepable and the rest probably too?
> All of them must be KF_TRUSTED_ARGS too.

Thanks, I'll include the trusted args flag. As to which are sleepable,
only memcg_flush_stats can block.

> 
>> +
>> +static const struct btf_kfunc_id_set bpf_memcontrol_kfunc_set = {
>> +       .owner          = THIS_MODULE,
>> +       .set            = &bpf_memcontrol_kfunc_ids,
>> +};
>> +
>> +static int __init bpf_memcontrol_kfunc_init(void)
>> +{
>> +       return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING,
>> +                                        &bpf_memcontrol_kfunc_set);
>> +}
> 
> Why tracing only?

Hmmm, initially I didn't think about use cases outside of the cgroup
iterator programs. After discussing with teammates though, some other
potential use cases could be within sched_ext or (future) bpf-oom. I'm
thinking I'll go with the "UNSPEC" type in v2.