[PATCH] blk-cgroup: skip rstat flush for cgroups without blkgs

[Usama Arif <usama.arif@linux.dev>]

Non-root io.stat reads flush the blkcg rstat subtree before walking
->blkg_list. This takes the subsystem rstat lock and checks for pending
updates on every possible CPU, even when there is no per-device state to
print.

Blkg creation is hierarchical. A descendant blkg implies an ancestor
blkg on the same device, so an empty ->blkg_list means the subtree cannot
contribute any output. Return early in that case.

A concurrent first blkg may be deferred to the next read. This is
consistent with the existing non-atomic flush and list walk.

This is not rare: 340 of 969 io.stat files were empty on a 316-CPU host.
In a 16-vCPU lockdep-enabled guest, the median
open/read/close time over 35 batches of 20,000 reads changed as follows:

                         before     after
  empty io.stat          49.1 us   41.0 us
  populated io.stat      49.9 us   50.2 us

This is a 16.6% reduction for empty reads. Populated reads were unchanged
within measurement noise.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 block/blk-cgroup.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 2b5c29434e426..0d7a451473b04 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1189,10 +1189,17 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 	struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
 	struct blkcg_gq *blkg;
 
-	if (!seq_css(sf)->parent)
+	if (!seq_css(sf)->parent) {
 		blkcg_fill_root_iostats();
-	else
+	} else {
+		/*
+		 * Descendant blkgs imply ancestor blkgs, so an empty
+		 * ->blkg_list has no stats to report.
+		 */
+		if (hlist_empty(&blkcg->blkg_list))
+			return 0;
 		css_rstat_flush(&blkcg->css);
+	}
 
 	guard(spinlock_irq)(&blkcg->lock);
 	hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node)
-- 
2.53.0-Meta