Re: [REGRESSION] io_uring/futex: scalar wait/wake slowdown after 079afb081c42

[Jens Axboe]

On 7/30/26 4:42 AM, Chengfeng Lin wrote:
> Hi Jens,
> 
> I tested 079afb081c42 against its direct parent on bare metal. In a narrow
> scalar io_uring futex wait/wake workload, the child was 9.27% slower. A
> separate 338-line standalone reproduced the result at 10.02%. All compared
> kernels actually ran with preempt=full.
> 
> #regzbot introduced: 079afb081c4288e94d5e4223d3eb6306d853c68b
> #regzbot title: io_uring scalar futex wait/wake slowdown
> 
> This is a focused synthetic microbenchmark, not an application benchmark. It
> uses one raw-UAPI ring and 32 cacheline-separated private futex words on one
> pinned P-core. Each timed cycle submits 32 scalar IORING_OP_FUTEX_WAIT
> requests, then 32 scalar IORING_OP_FUTEX_WAKE requests, and drains exactly 64
> CQEs. Every wait must return 0 and every wake must return 1.
> 
> I used a fresh boot for each point:
> 
>   6a8118a77eec parent A -> 079afb081c42 child -> 6a8118a77eec parent B
> 
> Each point had 3 warm-up rounds and 15 measured rounds. Every measured round
> ran 512 cycles, or 16,384 wait/wake pairs. The results in ns/pair were:
> 
>   implementation   parent A    child    parent B    child vs midpoint
>   formal              180.079  196.647     179.856        +9.268%
>   standalone          180.171  197.856     179.502       +10.020%
> 
> For the formal source, dropping the first measured round gave +9.269%. Parent
> drift was -0.124%, and the maximum CV was 0.169%. The standalone drop-first
> result was +9.996%, with -0.371% parent drift. All 90 scalar timing rows
> passed the CQE, result, timeout, overflow, outstanding-request, and CPU
> checks.
> 
> An untimed child trace also hit io_futex_prep(), io_futex_wait(),
> io_futex_wake(), and io_futex_complete() with the expected request counts.
> 
> A matched WAITV -> WAKE profile changed by only +1.385%, below my preregistered
> 5% signal gate, so my claim is limited to scalar wait/wake.
> 
> I understand that 079afb fixes the exit-time use-after-free by keeping pending
> private futex waits visible to cancellation before their mm state disappears.
> Scalar WAIT and WAKE both use io_futex_prep(), so in the child both sides of
> each measured pair execute the added tracking call. I am not suggesting a
> revert.
> 
> Is this per-request cost an expected trade-off for the lifetime fix, or could
> the same exit/mm-lifetime guarantee be retained with cheaper tracking?
> 
> Evidence bundle:
> 
>   https://github.com/lcf0399/linux-regression-evidence/tree/65c8cbf86f40cbe759e3f7db1d29d152ba03a8f2/io-uring-futex-inflight-wait-wake
> 
> Standalone reproducer:
> 
>   https://github.com/lcf0399/linux-regression-evidence/tree/65c8cbf86f40cbe759e3f7db1d29d152ba03a8f2/io-uring-futex-inflight-wait-wake/reproducer

Great report, thanks for that! I'll take a look at this. The inflight
tracking is a bit of a big hammer for sure for this, and it isn't even
needed on the wake side. Can you tell me what parameters you're using
for the reproducers?

-- 
Jens Axboe