Re: [PATCH] btrfs: use lockless read in nr_cached_objects shrinker callback

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, May 29, 2026 at 02:23:46PM -0700, Boris Burkov wrote:
> From: Ben Maurer <bmaurer@meta.com>
> 
> Under heavy memcg-driven slab reclaim with many memcgs and CPUs,
> shrink_slab_memcg() invokes the per-superblock count callback once
> per (memcg, NUMA node) tuple. For btrfs that callback reaches
> percpu_counter_sum_positive() on fs_info->evictable_extent_maps,
> which takes the percpu_counter's raw spinlock with IRQs disabled and
> walks every online CPU. With hundreds of memcgs driving reclaim on a
> host with dozens of CPUs, this counter lock becomes a global
> serialization point: profiles show CPU pinned in the spin_lock_irqsave
> acquire under __percpu_counter_sum, with cross-CPU IPIs hitting
> csd_lock_wait_toolong while waiting for spinning vCPUs.
> 
> The shrinker count is advisory -- super_cache_count() already notes
> "counts can change between super_cache_count and super_cache_scan, so
> we really don't need locks here." Use percpu_counter_read_positive(),
> which is lockless. Worst-case skew is bounded by batch * num_online_cpus
> (a few thousand), negligible compared to the millions of extent maps a
> busy filesystem accumulates and well within the noise that the shrinker
> already tolerates.
> 
> Tested-by: Boris Burkov <boris@bur.io>
> Signed-off-by: Ben Maurer <bmaurer@meta.com>

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>