From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A1887CCA47C for ; Thu, 23 Jun 2022 14:08:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231598AbiFWOIT (ORCPT ); Thu, 23 Jun 2022 10:08:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42864 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231865AbiFWOIP (ORCPT ); Thu, 23 Jun 2022 10:08:15 -0400 Received: from mail-il1-x12f.google.com (mail-il1-x12f.google.com [IPv6:2607:f8b0:4864:20::12f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 852FC3EBB2 for ; Thu, 23 Jun 2022 07:08:12 -0700 (PDT) Received: by mail-il1-x12f.google.com with SMTP id k18so8320026ilr.11 for ; Thu, 23 Jun 2022 07:08:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20210112.gappssmtp.com; s=20210112; h=message-id:date:mime-version:user-agent:subject:content-language:to :cc:references:from:in-reply-to:content-transfer-encoding; bh=KtGcO+WB7Oy31zJ1NLEXMcO/8OKl158blOykwTSsaFI=; b=4ce+zg40R4jacXjIuEcZL3uMH4IDI2M9HOnYW7wv98wkkvUhGuCydAJc6Se4pSM247 YymnVYnY14fJFGztOTW7f/d+DLVv4W++E/ZKYHCvd19ymdEMrStnJ2he+MYemMnr8sEC 7O3MOIZX+4jwvGlPVurEM0kKiPKKjle0LDKRzNO5wosRvusTZ44fKt78CGb8STI72Bwk W6dJgxK4O2iBPIjpl6h3I5cteN4U5UVF7iY9+qRtMtIwdTqWV0zrszbhI6E5Fo8nLakf 0g4tkV6Lk9NGh7XJHSncWACHPAbQJjnxna5SRvSDlBZEer/QzGDkDGQlkcm3E6ks7rre ecDA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:in-reply-to :content-transfer-encoding; bh=KtGcO+WB7Oy31zJ1NLEXMcO/8OKl158blOykwTSsaFI=; b=3jjoe2cRNVpFBBcXwkqMfhYuhdpzrw50mcaIqkz106/+bshI04NST3PWZ0ZFHV1Iiz VCfHdrdLWGPad6xBWTOgH7aA/ul6sOLbNKavaSfF3f+tHtn4aAKMo17mIEfxlyRuol6M GghNiHddHKdvGfFEkUhk3hskcOd2CTo7poJLQGxUdgHaiufg/V2jahjCFik0aLDEf6Oa 5b7LHfsa5eOJ5hjWlfiWuLtbzRpXk6T+PmyzU1KdYl1sJJt8OB33WSMdUMoGljgi6fL7 LRU7mnmFmb33/ZbmXiXybO2N46Ao6Fo8QS5v1wlwjpqlpUSjCYu7b0b8TPt+zvbYqH8v b95Q== X-Gm-Message-State: AJIora/aYe0Bv8Xj3Y1tjmM24c8wnyfpuy9/smKirF6otgKeeNoYvL6M u8e2o1BQsKdZjg3heVPw7JiovQ== X-Google-Smtp-Source: AGRyM1tRs77eZWKpqkJDyVpzGUbJApjrcKSQvsHy/XjJJqs71WQ46bBpWhN5tge/jwRdgJeElZXnWA== X-Received: by 2002:a05:6e02:1a89:b0:2d9:2feb:da69 with SMTP id k9-20020a056e021a8900b002d92febda69mr5397398ilv.189.1655993291618; Thu, 23 Jun 2022 07:08:11 -0700 (PDT) Received: from [192.168.1.172] ([207.135.234.126]) by smtp.gmail.com with ESMTPSA id s15-20020a02c7cf000000b0032b5e78bfcbsm9877639jao.135.2022.06.23.07.08.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Jun 2022 07:08:11 -0700 (PDT) Message-ID: <698e189e-834c-60b0-6cb8-fdad78cd0a49@kernel.dk> Date: Thu, 23 Jun 2022 08:08:08 -0600 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:91.0) Gecko/20100101 Thunderbird/91.10.0 Subject: Re: [RFC] a new way to achieve asynchronous IO Content-Language: en-US To: Hao Xu , io-uring Cc: Pavel Begunkov , dvernet@fb.com References: <3d1452da-ecec-fdc7-626c-bcd79df23c92@linux.dev> <1237fa26-3190-7c92-c516-9cf2a750fab4@linux.dev> From: Jens Axboe In-Reply-To: <1237fa26-3190-7c92-c516-9cf2a750fab4@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 6/23/22 7:31 AM, Hao Xu wrote: > On 6/20/22 21:41, Jens Axboe wrote: >> On 6/20/22 6:01 AM, Hao Xu wrote: >>> Hi, >>> I've some thought on the way of doing async IO. The current model is: >>> (given we are using SQPOLL mode) >>> >>> the sqthread does: >>> (a) Issue a request with nowait/nonblock flag. >>> (b) If it would block, reutrn -EAGAIN >>> (c) The io_uring layer captures this -EAGAIN and wake up/create >>> a io-worker to execute the request synchronously. >>> (d) Try to issue other requests in the above steps again. >>> >>> This implementation has two downsides: >>> (1) we have to find all the block point in the IO stack manually and >>> change them into "nowait/nonblock friendly". >>> (2) when we raise another io-worker to do the request, we submit the >>> request from the very beginning. This isn't a little bit inefficient. >>> >>> >>> While I think we can actually do it in a reverse way: >>> (given we are using SQPOLL mode) >>> >>> the sqthread1 does: >>> (a) Issue a request in the synchronous way >>> (b) If it is blocked/scheduled soon, raise another sqthread2 >>> (c) sqthread2 tries to issue other requests in the same way. >>> >>> This solves problem (1), and may solve (2). >>> For (1), we just do the sqthread waken-up at the beginning of schedule() >>> just like what the io-worker and system-worker do. No need to find all >>> the block point. >>> For (2), we continue the blocked request from where it is blocked when >>> resource is satisfied. >>> >>> What we need to take care is making sure there is only one task >>> submitting the requests. >>> >>> To achieve this, we can maintain a pool of sqthread just like the iowq. >>> >>> I've done a very simple/ugly POC to demonstrate this: >>> >>> https://github.com/HowHsu/linux/commit/183be142493b5a816b58bd95ae4f0926227b587b >>> >>> I also wrote a simple test to test it, which submits two sqes, one >>> read(pipe), one nop request. The first one will be block since no data >>> in the pipe. Then a new sqthread was created/waken up to submit the >>> second one and then some data is written to the pipe(by a unrelated >>> user thread), soon the first sqthread is waken up and continues the >>> request. >>> >>> If the idea sounds no fatal issue I'll change the POC to real patches. >>> Any comments are welcome! >> >> One thing I've always wanted to try out is kind of similar to this, but >> a superset of it. Basically io-wq isn't an explicit offload mechanism, >> it just happens automatically if the issue blocks. This applies to both >> SQPOLL and non-SQPOLL. >> >> This takes a page out of the old syslet/threadlet that Ingo Molnar did >> way back in the day [1], but it never really went anywhere. But the >> pass-on-block primitive would apply very nice to io_uring. > > I've read a part of the syslet/threadlet patchset, seems it has > something that I need, my first idea about the new iowq offload is > just like syslet----if blocked, trigger a new worker, deliver the > context to it, and then update the current context so that we return > to the place of sqe submission. But I just didn't know how to do it. Exactly, what you mentioned was very close to what I had considered in the past, and what the syslet/threadlet attempted to do. Except it flips it upside down a bit, which I do think is probably the saner way to do it rather than have the original block and fork a new one. > By the way, may I ask why the syslet/threadlet is not merged to the > mainline. The mail thread is very long, haven't gotten a chance to > read all of it. Not quite sure, it's been a long time. IMHO it's a good idea looking for the right interface, which we now have. So the time may be ripe to do something like this, finally. > > For the approach I posted, I found it is actually SQPOLL-nonrelated. > The original conext just wake up a worker in the pool to do the > submission, and if one blocks, another one wakes up to do the > submission. It is definitely easier to implement than something like > syslet(context delivery) since the new worker naturally goes to the > place of submission thus no context delivery needed. but a downside is > every time we call io_uring_enter to submit a batch of sqes, there is a > wakeup at the beginning. > > I'll try if I can implement a context delivery version. Sounds good, thanks. -- Jens Axboe