Re: [PATCH v3 RESEND] sched: Lift cgroup update locking to core to prevent CFS/SCX divergence

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 10, 2026 at 03:18:57PM +0000, Michal Blaszczyk wrote:
> Concurrent writes to cgroup control files (such as cpu.shares or
> cpu.weight) can lead to state divergence between CFS and SCX.
> 
> For instance, in cpu_shares_write_u64(), the CFS update is serialized
> by shares_mutex (internal to fair.c), but this lock is dropped before
> scx_group_set_weight() is called. The latter only acquires a read
> semaphore (scx_cgroup_ops_rwsem), allowing multiple threads to evaluate
> and act on the sched_ext update concurrently.
> 
> This serialization gap allows concurrent writes to interleave.
> As a result, the recorded state in CFS, the SCX internal bookkeeping
> (e.g., tg->scx.weight), and the BPF scheduler itself can end up operating
> on completely distinct parameters (pairwise distinct values).
> 
> Similar races are present in tg_set_bandwidth(), cpu_idle_write_s64(),
> cpu_weight_write_u64(), and cpu_weight_nice_write_s64().
> 
> Fix this by moving the CFS locking up into the core layer in
> `kernel/sched/core.c`. By acquiring these locks directly in the core
> write handlers, both the CFS and SCX callbacks are executed atomically
> under the same lock.
> 
> Fixes: 819513666966 ("sched_ext: Add cgroup support")
> Signed-off-by: Michal Blaszczyk <michalblk@google.com>
> Reviewed-by: Andrea Righi <arighi@nvidia.com>
> Acked-by: Tejun Heo <tj@kernel.org>
> ---

Thanks!