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Fri, 23 May 2025 08:20:13 -0700 (PDT) Message-ID: <7bf620dc-1b5c-4401-a03c-16978de0598a@kernel.dk> Date: Fri, 23 May 2025 09:20:12 -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> From: Jens Axboe Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. -- Jens Axboe