Re: [PATCH bpf-next v1 4/6] mm: introduce BPF kfuncs to access memcg statistics and events

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Dec 18, 2025 at 05:57:48PM -0800, Roman Gushchin wrote:
> Introduce BPF kfuncs to conveniently access memcg data:
>   - bpf_mem_cgroup_vm_events(),
>   - bpf_mem_cgroup_usage(),
>   - bpf_mem_cgroup_page_state(),
>   - bpf_mem_cgroup_flush_stats().
> 
> These functions are useful for implementing BPF OOM policies, but
> also can be used to accelerate access to the memcg data. Reading
> it through cgroupfs is much more expensive, roughly 5x, mostly
> because of the need to convert the data into the text and back.
> 
> JP Kobryn:
> An experiment was setup to compare the performance of a program that
> uses the traditional method of reading memory.stat vs a program using
> the new kfuncs. The control program opens up the root memory.stat file
> and for 1M iterations reads, converts the string values to numeric data,
> then seeks back to the beginning. The experimental program sets up the
> requisite libbpf objects and for 1M iterations invokes a bpf program
> which uses the kfuncs to fetch all available stats for node_stat_item,
> memcg_stat_item, and vm_event_item types.
> 
> The results showed a significant perf benefit on the experimental side,
> outperforming the control side by a margin of 93%. In kernel mode,
> elapsed time was reduced by 80%, while in user mode, over 99% of time
> was saved.
> 
> control: elapsed time
> real    0m38.318s
> user    0m25.131s
> sys     0m13.070s
> 
> experiment: elapsed time
> real    0m2.789s
> user    0m0.187s
> sys     0m2.512s
> 
> control: perf data
> 33.43% a.out libc.so.6         [.] __vfscanf_internal
>  6.88% a.out [kernel.kallsyms] [k] vsnprintf
>  6.33% a.out libc.so.6         [.] _IO_fgets
>  5.51% a.out [kernel.kallsyms] [k] format_decode
>  4.31% a.out libc.so.6         [.] __GI_____strtoull_l_internal
>  3.78% a.out [kernel.kallsyms] [k] string
>  3.53% a.out [kernel.kallsyms] [k] number
>  2.71% a.out libc.so.6         [.] _IO_sputbackc
>  2.41% a.out [kernel.kallsyms] [k] strlen
>  1.98% a.out a.out             [.] main
>  1.70% a.out libc.so.6         [.] _IO_getline_info
>  1.51% a.out libc.so.6         [.] __isoc99_sscanf
>  1.47% a.out [kernel.kallsyms] [k] memory_stat_format
>  1.47% a.out [kernel.kallsyms] [k] memcpy_orig
>  1.41% a.out [kernel.kallsyms] [k] seq_buf_printf
> 
> experiment: perf data
> 10.55% memcgstat bpf_prog_..._query [k] bpf_prog_16aab2f19fa982a7_query
>  6.90% memcgstat [kernel.kallsyms]  [k] memcg_page_state_output
>  3.55% memcgstat [kernel.kallsyms]  [k] _raw_spin_lock
>  3.12% memcgstat [kernel.kallsyms]  [k] memcg_events
>  2.87% memcgstat [kernel.kallsyms]  [k] __memcg_slab_post_alloc_hook
>  2.73% memcgstat [kernel.kallsyms]  [k] kmem_cache_free
>  2.70% memcgstat [kernel.kallsyms]  [k] entry_SYSRETQ_unsafe_stack
>  2.25% memcgstat [kernel.kallsyms]  [k] __memcg_slab_free_hook
>  2.06% memcgstat [kernel.kallsyms]  [k] get_page_from_freelist
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> Co-developed-by: JP Kobryn <inwardvessel@gmail.com>
> Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---

[...]

> +
> +/**
> + * bpf_mem_cgroup_usage - Read memory cgroup's usage
> + * @memcg: memory cgroup
> + *
> + * Returns current memory cgroup size in bytes.
> + */
> +__bpf_kfunc unsigned long bpf_mem_cgroup_usage(struct mem_cgroup *memcg)
> +{
> +	return page_counter_read(&memcg->memory) * PAGE_SIZE;

Similar to mem_cgroup_usage() should we special handle root memcg?

Otherwise looks good to me.