btrfs: use lockless read in nr_cached_objects shrinker callback

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>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Ben Maurer <bmaurer@meta.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 958dd185c0d6c557df23b985c4a50351f2de170a..c946bccf07481baa56e8d2d9e08c22a0bd2de254 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2432,7 +2432,7 @@ static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
 static long btrfs_nr_cached_objects(struct super_block *sb, struct shrink_control *sc)
 {
        struct btrfs_fs_info *fs_info = btrfs_sb(sb);
-       const s64 nr = percpu_counter_sum_positive(&fs_info->evictable_extent_maps);
+       const s64 nr = percpu_counter_read_positive(&fs_info->evictable_extent_maps);
 
        trace_btrfs_extent_map_shrinker_count(fs_info, nr);