Re: [PATCH v2] io_uring: add IORING_SETUP_SQTHREAD_STATS flag to enable sqthread stats collection

[Jens Axboe <axboe@kernel.dk>]

On 10/20/25 5:30 AM, Fengnan Chang wrote:
> In previous versions, getrusage was always called in sqrthread
> to count work time, but this could incur some overhead.
> This patch turn off stats by default, and introduces a new flag
> IORING_SETUP_SQTHREAD_STATS that allows user to enable the
> collection of statistics in the sqthread.
> 
> ./t/io_uring -p1 -d128 -b4096 -s32 -c1 -F1 -B1 -R1 -X1 -n1 ./testfile
> IOPS base: 570K, patch: 590K
> 
> ./t/io_uring -p1 -d128 -b4096 -s32 -c1 -F1 -B1 -R1 -X1 -n1 /dev/nvme1n1
> IOPS base: 826K, patch: 889K

That's a crazy amount of overhead indeed. I'm assuming this is
because the task has lots of threads? And/or going through the retry
a lot? Might make more sense to have a cheaper and more rough
getrusage() instead? All we really use is ru_stime.{tv_sec,tv_nsec}.
Should it be using RUSAGE_THREAD? Correct me if I'm wrong, but using
PTHREAD_SELF actually seems wrong as-is.

In any case, I don't think a setup flag is the right choice here. That
space is fairly limited, and SQPOLL is also a bit of an esoteric
feature. Hence not sure a setup flag is the right approach. Would
probably make more sense to have a REGISTER opcode to get/set various
features like this, I'm sure it's not the last thing like this we'll run
into. But as mentioned, I do think just having a more light weight
getrusage would perhaps be prudent.

-- 
Jens Axboe