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([2620:10d:c090:500::6:c573]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-28e8d1dc00esm140825475ad.117.2025.10.06.13.33.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 06 Oct 2025 13:33:08 -0700 (PDT) Message-ID: <82c392dc-e750-43d9-9394-1df00a366ae0@gmail.com> Date: Mon, 6 Oct 2025 13:33:07 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: JP Kobryn Subject: Re: [PATCH] memcg: introduce kfuncs for fetching memcg stats To: Alexei Starovoitov Cc: Shakeel Butt , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Yosry Ahmed , Johannes Weiner , Tejun Heo , Andrew Morton , LKML , "open list:CONTROL GROUP (CGROUP)" , linux-mm , bpf , Kernel Team References: <20251001045456.313750-1-inwardvessel@gmail.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10/1/25 3:25 PM, Alexei Starovoitov wrote: > On Tue, Sep 30, 2025 at 9:57 PM JP Kobryn 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.