Re: [PATCH] cgroup/rstat: change cgroup_base_stat to atomic

[Michal Koutný]

Hello.

This is interesting.

On Tue, Jun 17, 2025 at 12:26:45PM +0200, Bertrand Wlodarczyk <bertrand.wlodarczyk@xxxxxxxxx> wrote:
> The kernel currently faces scalability issues when multiple userspace
> programs attempt to read cgroup statistics concurrently.

Does "currently" mean that you didn't observe this before per-subsys
split?

> The primary bottleneck is the css_cgroup_lock in cgroup_rstat_flush,
> which prevents access and updates to the statistics
> of the css from multiple CPUs in parallel.

I think this description needs some refresh on top of the current
mainline (at least after the commit 748922dcfabdd ("cgroup: use
subsystem-specific rstat locks to avoid contention") to be clear which
lock (and locking functions) is apparently contentious.

> Given that rstat operates on a per-CPU basis and only aggregates
> statistics in the parent cgroup, there is no compelling reason
> why these statistics cannot be atomic.
> By eliminating the lock, each CPU can traverse its rstat hierarchy
> independently, without blocking. Synchronization is achieved during
> parent propagation through atomic operations.
> 
> This change significantly enhances performance in scenarios
> where multiple CPUs access CPU rstat within a single cgroup hierarchy,
> yielding a performance improvement of around 50 times compared
> to the mainline version.

Given the recent changes and to be on the same page, it'd be better if
you referred to particular commits/tags when benchmarking so that it's
unambiguous what is compared with what.

> Notably, performance for memory and I/O rstats remains unchanged,
> as these are managed in separate submodules.
 

> Additionally, this patch addresses a race condition detectable
> in the current mainline by KCSAN in __cgroup_account_cputime,
> which occurs when attempting to read a single hierarchy
> from multiple CPUs.

Could you please extract this fix and send it separately?

Thanks,
Michal

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