Re: [PATCH v2 3/3] mm: memcg: skip the RCU lock when the memcg is not dying

[Shakeel Butt]

On Mon, Aug 31, 2026 at 05:46:11PM +0800, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@xxxxxxxxxx>
> 
> get_non_dying_memcg_start() takes rcu_read_lock() on every stat update, but
> the lock only protects the upward walk to a non-dying ancestor, which
> happens solely while a memcg is being offlined.  The dying check itself
> reads the CSS_DYING flag of a memcg the caller already holds a reference
> to, so it is safe without the lock.
> 
> Check memcg_is_dying() first and return immediately when the memcg is
> alive, taking the RCU lock only on the rare dying path.  On an anon
> fault/charge churn workload in a memcg this recovers the ~0.6% overhead
> added by the previous patch (4368077 vs 4343159 pages/s before, back to
> ~4377000 pages/s after).
> 
> Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>

Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>