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

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

On 17/09/2026 17:31, Tejun Heo wrote:
> 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.
> 

The updated numbers without lockdep are:

                             before    after
      empty io.stat           7.02 us   5.47 us
      populated io.stat       7.84 us   7.83 us

    The median paired reduction for empty reads was 22.1% (20.0-22.4% across
    pairs). System CPU time per read fell by 22.8%. Populated reads showed no
    consiste



>> 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>
> 
> That said, I don't see any downsides:
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> 
> Thanks.
>