Re: [PATCH v5 0/7] move stock from mem_cgroup to page_counter

[Joshua Hahn]

> v4 --> v5
> =========
> - The stock is now a raw_spinlock_t and an unsigned long to more closely
>   match the original semantics of the stock code.
> - Draining is asynchronous again, we add a work_struct per-page_counter
>   (not percpu) that walks every cpu. This eliminates the concerns
>   of doing a synchronous drain.
> - page_counter_try_charge transparently handles stock.
> - Addressed the netperf regression by reworking the refill path to match
>   the vanilla uncharge path more closely.
> - Correctness fixes for the percpu pointer access usage
> - More testing to demonstrate that this series achieves its goal.
> - Included Shakeel's stock watermarks from [1].
> - Wordsmithing
> 
> INTRO
> =====
> Memcg currently keeps a "stock" of 64 pages per-cpu to cache pre-charged
> allocations, allowing small and frequent allocations to avoid walking
> the expensive mem_cgroup hierarchy traversal each time. This fastpath
> offers real improvements, but there is room for improvement:
> 1. Currently, each CPU tracks up to 7 (NR_MEMCG_STOCK) mem_cgroups. When
>    more than 7 mem_cgroups have stock present on a single CPU, a random
>    victim is evicted and its associated stock is drained.
> 2. When one cgroup runs out of memory and needs to drain stock across
>    all CPUs it has stock cached in, those CPUs will drain all other
>    memcgs' stock present in that CPU. This leads to inefficient stock
>    caching and cross-memcg interference under memory pressure.
> 3. Stock management is tightly coupled to struct mem_cgroup, which makes
>    it difficult to add a new page_counter to mem_cgroup and have
>    multiple sources of stock management.
> 
> This series moves the per-cpu stock down into page_counter, so that
> page_counter_try_charge() transparently serves a charge from the stock
> and refills it, and each counter owns and drains its own cache. This
> eliminates the 7 memcg-per-cpu slot limit, the random cross-memcg stock
> drains, and the slot traversal.
> 
> In turn, we can add independent stock management for additional
> page_counters in each memcg, which is used in my tiered memory limits
> series to add a new page_counter to track toptier usage [2]. Patch 7
> uses it to give memsw its own stock.
> 
> Because the stock is now a property of the counter rather than of the
> cpu, it is also reachable remotely, so draining no longer has to run on
> the cpu that owns the cache.
> 
> This series preserves as much of the old semantics as possible,
> including non-spinning safety by using trylocks for stock access.
> The old !allow_spinning semantics in try_charge_memcg are slightly
> different now though; outside NMI, page_counter_try_charge may perform
> a speculative batch charge and a refill.

Hello reviewers,

I just wanted to note that Sashiko seems to have raised no concerns [1]
with this series : -)

Thank you for your feedback and input!!
Joshua

[1] https://sashiko.dev/#/patchset/20260831163752.2193337-1-joshua.hahnjy%40gmail.com