blk-cgroup: skip rstat flush for cgroups without blkgs

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 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 <tj@kernel.org>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions