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

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On 7/30/26 8:34 AM, Jens Axboe wrote:
> 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?

Try with these two patches.

-- 
Jens Axboe

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