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Wed, 28 May 2025 06:39:36 -0700 (PDT) Message-ID: <84ac5b93-7f1c-4092-80a8-9f9813cdbc1b@kernel.dk> Date: Wed, 28 May 2025 07:39:36 -0600 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [External] Re: [RFC PATCH] io_uring: fix io worker thread that keeps creating and destroying To: Fengnan Chang Cc: asml.silence@gmail.com, io-uring@vger.kernel.org, Diangang Li References: <20250522090909.73212-1-changfengnan@bytedance.com> <356c5068-bd97-419a-884c-bcdb04ad6820@kernel.dk> <7bf620dc-1b5c-4401-a03c-16978de0598a@kernel.dk> From: Jens Axboe Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/26/25 5:14 AM, Fengnan Chang wrote: > Jens Axboe ?2025?5?23??? 23:20??? >> >> On 5/23/25 1:57 AM, Fengnan Chang wrote: >>> Jens Axboe ?2025?5?22??? 22:29??? >>>> >>>> On 5/22/25 6:01 AM, Fengnan Chang wrote: >>>>> Jens Axboe ?2025?5?22??? 19:35??? >>>>>> >>>>>> On 5/22/25 3:09 AM, Fengnan Chang wrote: >>>>>>> When running fio with buffer io and stable iops, I observed that >>>>>>> part of io_worker threads keeps creating and destroying. >>>>>>> Using this command can reproduce: >>>>>>> fio --ioengine=io_uring --rw=randrw --bs=4k --direct=0 --size=100G >>>>>>> --iodepth=256 --filename=/data03/fio-rand-read --name=test >>>>>>> ps -L -p pid, you can see about 256 io_worker threads, and thread >>>>>>> id keeps changing. >>>>>>> And I do some debugging, most workers create happen in >>>>>>> create_worker_cb. In create_worker_cb, if all workers have gone to >>>>>>> sleep, and we have more work, we try to create new worker (let's >>>>>>> call it worker B) to handle it. And when new work comes, >>>>>>> io_wq_enqueue will activate free worker (let's call it worker A) or >>>>>>> create new one. It may cause worker A and B compete for one work. >>>>>>> Since buffered write is hashed work, buffered write to a given file >>>>>>> is serialized, only one worker gets the work in the end, the other >>>>>>> worker goes to sleep. After repeating it many times, a lot of >>>>>>> io_worker threads created, handles a few works or even no work to >>>>>>> handle,and exit. >>>>>>> There are several solutions: >>>>>>> 1. Since all work is insert in io_wq_enqueue, io_wq_enqueue will >>>>>>> create worker too, remove create worker action in create_worker_cb >>>>>>> is fine, maybe affect performance? >>>>>>> 2. When wq->hash->map bit is set, insert hashed work item, new work >>>>>>> only put in wq->hash_tail, not link to work_list, >>>>>>> io_worker_handle_work need to check hash_tail after a whole dependent >>>>>>> link, io_acct_run_queue will return false when new work insert, no >>>>>>> new thread will be created either in io_wqe_dec_running. >>>>>>> 3. Check is there only one hash bucket in io_wqe_dec_running. If only >>>>>>> one hash bucket, don't create worker, io_wq_enqueue will handle it. >>>>>> >>>>>> Nice catch on this! Does indeed look like a problem. Not a huge fan of >>>>>> approach 3. Without having really looked into this yet, my initial idea >>>>>> would've been to do some variant of solution 1 above. io_wq_enqueue() >>>>>> checks if we need to create a worker, which basically boils down to "do >>>>>> we have a free worker right now". If we do not, we create one. But the >>>>>> question is really "do we need a new worker for this?", and if we're >>>>>> inserting hashed worked and we have existing hashed work for the SAME >>>>>> hash and it's busy, then the answer should be "no" as it'd be pointless >>>>>> to create that worker. >>>>> >>>>> Agree >>>>> >>>>>> >>>>>> Would it be feasible to augment the check in there such that >>>>>> io_wq_enqueue() doesn't create a new worker for that case? And I guess a >>>>>> followup question is, would that even be enough, do we always need to >>>>>> cover the io_wq_dec_running() running case as well as >>>>>> io_acct_run_queue() will return true as well since it doesn't know about >>>>>> this either? >>>>> Yes?It is feasible to avoid creating a worker by adding some checks in >>>>> io_wq_enqueue. But what I have observed so far is most workers are >>>>> created in io_wq_dec_running (why no worker create in io_wq_enqueue? >>>>> I didn't figure it out now), it seems no need to check this >>>>> in io_wq_enqueue. And cover io_wq_dec_running is necessary. >>>> >>>> The general concept for io-wq is that it's always assumed that a worker >>>> won't block, and if it does AND more work is available, at that point a >>>> new worker is created. io_wq_dec_running() is called by the scheduler >>>> when a worker is scheduled out, eg blocking, and then an extra worker is >>>> created at that point, if necessary. >>>> >>>> I wonder if we can get away with something like the below? Basically two >>>> things in there: >>>> >>>> 1) If a worker goes to sleep AND it doesn't have a current work >>>> assigned, just ignore it. Really a separate change, but seems to >>>> conceptually make sense - a new worker should only be created off >>>> that path, if it's currenly handling a work item and goes to sleep. >>>> >>>> 2) If there is current work, defer if it's hashed and the next work item >>>> in that list is also hashed and of the same value. >>> I like this change, this makes the logic clearer. This patch looks good, >>> I'll do more tests next week. >> >> Thanks for taking a look - I've posted it as a 3 patch series, as 1+2 >> above are really two separate things that need sorting imho. I've queued >> it up for the next kernel release, so please do test next week when you >> have time. > > I have completed the test and the results are good. Thanks for re-testing! > But I still have a concern. When using one uring queue to buffer write > multiple files, previously there were multiple workers working, this > change will make only one worker working, which will reduce some > concurrency and may cause performance degradation. But I didn't find > it in the actual test, so my worry may be unnecessary. Could be one of two things: 1) None of the workers _actually_ end up blocking, in which case it's working as-designed. 2) We're now missing cases where we should indeed create a worker. This is a bug. I'll run some specific testing for io-wq here to see if it's 1 or 2. -- Jens Axboe