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

[Tejun Heo]

On Thu, Sep 17, 2026 at 09:10:20AM -0700, Usama Arif wrote:
> 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

Can you please test on a kernel w/ lockdep disabled? It doesn't make much
sense to compare performance numbers with lockdep enabled.

> 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@xxxxxxxxx>

That said, I don't see any downsides:

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun