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. i This is not rare: 340 of 969 io.stat files were empty on a 316-CPU host. In a 16-vCPU guest, the median open/read/close time over 35 batches of 20,000 reads changed as follows: before after empty io.stat 7.02 us 5.47 us populated io.stat 7.84 us 7.83 us This is a ~22% reduction for empty reads. Populated reads were unchanged. Acked-by: Tejun Heo Signed-off-by: Usama Arif --- v1 -> v2: - Updated commit message with perf numbers with lockdep disabled (Tejun) --- 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