From: Avi Kivity Subject: Re: [PATCH 0/8 v2] Non-blocking AIO Date: Mon, 6 Mar 2017 18:59:48 +0200 Message-ID: <56ae3a64-5e27-d7d4-5ab5-f5f68eef8b78@scylladb.com> References: <20170228233610.25456-1-rgoldwyn@suse.de> <347d19cb-dbb8-1d4f-dfb5-d1dd820dd65d@scylladb.com> <20170306082546.GA14932@quack2.suse.cz> <9b64c78e-c984-cf29-8f79-c48332a4c450@scylladb.com> <57c873b2-fed6-e717-fc4e-ed2e328173b6@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: Goldwyn Rodrigues , jack@suse.com, hch@infradead.org, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org To: Jens Axboe , Jan Kara Return-path: In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 03/06/2017 06:08 PM, Jens Axboe wrote: > On 03/06/2017 08:59 AM, Avi Kivity wrote: >> On 03/06/2017 05:38 PM, Jens Axboe wrote: >>> On 03/06/2017 08:29 AM, Avi Kivity wrote: >>>> On 03/06/2017 05:19 PM, Jens Axboe wrote: >>>>> On 03/06/2017 01:25 AM, Jan Kara wrote: >>>>>> On Sun 05-03-17 16:56:21, Avi Kivity wrote: >>>>>>>> The goal of the patch series is to return -EAGAIN/-EWOULDBLOCK if >>>>>>>> any of these conditions are met. This way userspace can push most >>>>>>>> of the write()s to the kernel to the best of its ability to complete >>>>>>>> and if it returns -EAGAIN, can defer it to another thread. >>>>>>>> >>>>>>> Is it not possible to push the iocb to a workqueue? This will allow >>>>>>> existing userspace to work with the new functionality, unchanged. Any >>>>>>> userspace implementation would have to do the same thing, so it's not like >>>>>>> we're saving anything by pushing it there. >>>>>> That is not easy because until IO is fully submitted, you need some parts >>>>>> of the context of the process which submits the IO (e.g. memory mappings, >>>>>> but possibly also other credentials). So you would need to somehow transfer >>>>>> this information to the workqueue. >>>>> Outside of technical challenges, the API also needs to return EAGAIN or >>>>> start blocking at some point. We can't expose a direct connection to >>>>> queue work like that, and let any user potentially create millions of >>>>> pending work items (and IOs). >>>> You wouldn't expect more concurrent events than the maxevents parameter >>>> that was supplied to io_setup syscall; it should have reserved any >>>> resources needed. >>> Doesn't matter what limit you apply, my point still stands - at some >>> point you have to return EAGAIN, or block. Returning EAGAIN without >>> the caller having flagged support for that change of behavior would >>> be problematic. >> Doesn't it already return EAGAIN (or some other error) if you exceed >> maxevents? > It's a setup thing. We check these limits when someone creates an IO > context, and carve out the specified entries form our global pool. Then > we free those "resources" when the io context is freed. > > Right now I can setup an IO context with 1000 entries on it, yet that > number has NO bearing on when io_submit() would potentially block or > return EAGAIN. > > We can have a huge gap on the intent signaled by io context setup, and > the reality imposed by what actually happens on the IO submission side. Isn't that a bug? Shouldn't that 1001st incomplete io_submit() return EAGAIN? Just tested it, and maxevents is not respected for this: io_setup(1, [0x7fc64537f000]) = 0 io_submit(0x7fc64537f000, 10, [{pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}]) = 10 which is unexpected, to me.