Re: [RFC PATCH bpf-next v7 00/11] mm: BPF struct_ops for dynamic memory protection and async reclaim

[Michal Hocko <mhocko@suse.com>]

On Tue 26-05-26 10:20:00, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@kylinos.cn>
> 
> Overview:
> This series introduces BPF struct_ops support for the memory controller,
> enabling userspace BPF programs to implement custom, dynamic memory
> management policies per cgroup. The feature allows BPF programs to hook
> into the core reclaim and charge paths without requiring kernel
> modifications, providing a flexible alternative to static knobs such as
> memory.low and memory.min.
>  
> The series enables two complementary use cases.
>  
> Dynamic memory protection: static memory protection thresholds
> (memory.low, memory.min) are poor fits for workloads whose actual memory
> activity varies over time. A high-priority cgroup holding a large working
> set but temporarily idle will still suppress reclaim on its siblings,
> wasting available memory. A BPF-driven approach can observe real workload
> activity -- page faults, charge/uncharge events -- and activate or
> withdraw protection dynamically.

Why the same cannot be achieved by dynamically changing protection?

> The test results at the end of this
> letter quantify the difference: in a scenario where the high-priority
> cgroup is idle, the BPF-controlled low-priority cgroup achieves roughly
> 37x higher throughput than with static memory.low.
>  
> Asynchronous proactive reclaim: the memcg_charged and memcg_uncharged
> hooks, combined with the BPF workqueue mechanism and the new
> bpf_try_to_free_mem_cgroup_pages() kfunc, enable BPF programs to perform
> proactive background reclaim without blocking the charge path. The
> pattern works as follows: the memcg_charged callback tracks accumulated
> memory usage; when usage crosses a configurable threshold, it enqueues an
> asynchronous work item via bpf_wq_start() and returns immediately without
> throttling the charging task. The workqueue callback then invokes
> bpf_try_to_free_mem_cgroup_pages() to reclaim pages from the target
> cgroup; if usage remains elevated after reclaim, the callback re-enqueues
> itself to continue. This allows a BPF program to keep a cgroup's
> footprint below its hard limit (memory.max) entirely in the background,
> avoiding the OOM killer or direct-reclaim stalls that would otherwise
> occur.

How do you account the overall work done to the specific memcg as the
large part of the reclaim is done from WQ context?
Also when introducing a BPF hook please focus on describing why existing
interfaces fail to achieve what you need. For the async reclaim why it
is not practical or feasible to use userspace driven memory reclaim.
-- 
Michal Hocko
SUSE Labs