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 X-Spam-Level: X-Spam-Status: No, score=-6.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1BCACC432C0 for ; Fri, 22 Nov 2019 10:32:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EDBD420726 for ; Fri, 22 Nov 2019 10:32:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726620AbfKVKc1 convert rfc822-to-8bit (ORCPT ); Fri, 22 Nov 2019 05:32:27 -0500 Received: from smtpbg501.qq.com ([203.205.250.101]:56768 "EHLO smtpbg.qq.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727648AbfKVKcZ (ORCPT ); Fri, 22 Nov 2019 05:32:25 -0500 X-QQ-mid: Xesmtp2t1574418438tiax3uvwp Received: from [172.16.31.194] (unknown [218.76.23.26]) by esmtp4.qq.com (ESMTP) with id ; Fri, 22 Nov 2019 18:26:47 +0800 (CST) X-QQ-SSF: 00000000000000B0SF101F00000000K X-QQ-FEAT: fs34Pe/+C2TOstQJolLBPB+L5GsarrxtdqzULLh2wXW8wRU6SesXnQgQ5dSkR IHIIzX+1x6lfcnf2eOA7Rnr68OSiRfS3uuW4WubV+Mx0A/xU4tFacLkAh5nDvhh9a2TFHy8 52YVN6MwMblWG2rkBX0lrhGi5AU6qGyvRuSKcRTdKT8RazEbJn9D5V1Gir1/Tkq/xjSz9iv YtMRO4xACgXH8znWyh6w+PGe6TqFWRUrMeEFp182PSN8NfCqSO1mVAnrVxpHdzElT7k9RCt x5Jxnb5AP8emxw6X/FUNqtOtExi5y7lSOCRX1wEhcx41xOsB7YCwILr/8= X-QQ-GoodBg: 0 Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 13.0 \(3601.0.10\)) Subject: Re: [PATCH RESEND] io_uring: a small optimization for REQ_F_DRAIN_LINK From: JackieLiu In-Reply-To: <4132c196-afd5-4ac8-0842-2746cc9e4a6f@gmail.com> Date: Fri, 22 Nov 2019 18:26:46 +0800 Cc: axboe@kernel.dk, io-uring@vger.kernel.org Content-Transfer-Encoding: 8BIT Message-Id: <01E7FC11-C19D-40FE-A471-5E1FAD2ED3D8@byteisland.com> References: <20191122060129.40251-1-jackieliu@byteisland.com> <20191122060129.40251-2-jackieliu@byteisland.com> <4132c196-afd5-4ac8-0842-2746cc9e4a6f@gmail.com> To: Pavel Begunkov X-Mailer: Apple Mail (2.3601.0.10) X-QQ-SENDSIZE: 520 Feedback-ID: Xesmtp:byteisland.com:bgweb:bgweb1 Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org > 2019年11月22日 18:05，Pavel Begunkov 写道： > > On 11/22/2019 9:01 AM, Jackie Liu wrote: >> From: Jackie Liu >> >> We don't need to set drain_next every time, make a small judgment >> and add unlikely, it looks like there will be a little optimization. >> > Not sure about that. It's 1 CMP + 1 SETcc/STORE, which works pretty fast > as @drain_next is hot (especially after read) and there is no write-read > dependency close. For yours, there is likely always 3 CMPs in the way. > > Did you benchmarked it somehow or compared assembly? It is only theoretically possible. In most cases, our drain_link and drain_next are both false, so only two CMPs are needed, and modern CPUs have branch predictions. Perhaps these judgments can be optimized. Your code is very nice, when I reading and understanding your code, I want to try if there is any other way to optimize it. Sometimes you don't need to reset drain_next, such as drain_link == true && drain_next == true, you don't need to set below one more time. -- Jackie Liu > >> Signed-off-by: Jackie Liu >> --- >> resend that patch, because reject by mail-list. >> >> fs/io_uring.c | 6 ++++-- >> 1 file changed, 4 insertions(+), 2 deletions(-) >> >> diff --git a/fs/io_uring.c b/fs/io_uring.c >> index 013e5ed..f4ec44a 100644 >> --- a/fs/io_uring.c >> +++ b/fs/io_uring.c >> @@ -2938,12 +2938,14 @@ static void __io_queue_sqe(struct io_kiocb *req) >> static void io_queue_sqe(struct io_kiocb *req) >> { >> int ret; >> + bool drain_link = req->flags & REQ_F_DRAIN_LINK; >> >> - if (unlikely(req->ctx->drain_next)) { >> + if (unlikely(req->ctx->drain_next && !drain_link)) { >> req->flags |= REQ_F_IO_DRAIN; >> req->ctx->drain_next = false; >> + } else if (unlikely(drain_link)) { >> + req->ctx->drain_next = true; >> } >> - req->ctx->drain_next = (req->flags & REQ_F_DRAIN_LINK); >> >> ret = io_req_defer(req); >> if (ret) { >> > > -- > Pavel Begunkov