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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=unavailable 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 16FE3C56202 for ; Wed, 25 Nov 2020 14:16:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B4C1A2087C for ; Wed, 25 Nov 2020 14:15:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729889AbgKYOPv (ORCPT ); Wed, 25 Nov 2020 09:15:51 -0500 Received: from out30-42.freemail.mail.aliyun.com ([115.124.30.42]:45643 "EHLO out30-42.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729109AbgKYOPv (ORCPT ); Wed, 25 Nov 2020 09:15:51 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R131e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0UGWQ0Kg_1606313739; Received: from 30.0.173.120(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0UGWQ0Kg_1606313739) by smtp.aliyun-inc.com(127.0.0.1); Wed, 25 Nov 2020 22:15:39 +0800 Subject: Re: [PATCH 4/7] blk-iocost: Add a flag to indicate if need update hwi To: Tejun Heo Cc: axboe@kernel.dk, baolin.wang7@gmail.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: From: Baolin Wang Message-ID: Date: Wed, 25 Nov 2020 22:15:38 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=gbk; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Hello, > > On Tue, Nov 24, 2020 at 11:33:33AM +0800, Baolin Wang wrote: >> @@ -1445,7 +1447,8 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt, >> * after the above debt payment. >> */ >> ctx.vbudget = vbudget; >> - current_hweight(iocg, NULL, &ctx.hw_inuse); >> + if (need_update_hwi) >> + current_hweight(iocg, NULL, &ctx.hw_inuse); > > So, if you look at the implementation of current_hweight(), it's > > 1. If nothing has changed, read out the cached values. > 2. If something has changed, recalculate. Yes, correct. > > and the "something changed" test is single memory read (most likely L1 hot > at this point) and testing for equality. IOW, the change you're suggesting > isn't much of an optimization. Maybe the compiler can do a somewhat better > job of arranging the code and it's a register load than memory load but > given that it's already a relatively cold wait path, this is unlikely to > make any actual difference. And that's how current_hweight() is meant to be > used. What I want to avoid is the 'atomic_read(&ioc->hweight_gen)' in current_hweight(), cause this is not a register load and is always a memory load. But introducing a flag can be cached and more light than a memory load. But after thinking more, I think we can just move the "current_hweight(iocg, NULL, &ctx.hw_inuse);" to the correct place without introducing new flag to optimize the code. How do you think the below code? diff --git a/block/blk-iocost.c b/block/blk-iocost.c index bbe86d1..db29200 100644 --- a/block/blk-iocost.c +++ b/block/blk-iocost.c @@ -1413,7 +1413,7 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt, lockdep_assert_held(&iocg->waitq.lock); - current_hweight(iocg, &hwa, NULL); + current_hweight(iocg, &hwa, &ctx.hw_inuse); vbudget = now->vnow - atomic64_read(&iocg->vtime); /* pay off debt */ @@ -1428,6 +1428,11 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt, atomic64_add(vpay, &iocg->done_vtime); iocg_pay_debt(iocg, abs_vpay, now); vbudget -= vpay; + /* + * As paying off debt restores hw_inuse, it must be read after + * the above debt payment. + */ + current_hweight(iocg, NULL, &ctx.hw_inuse); } if (iocg->abs_vdebt || iocg->delay) @@ -1446,11 +1451,9 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt, /* * Wake up the ones which are due and see how much vtime we'll need for - * the next one. As paying off debt restores hw_inuse, it must be read - * after the above debt payment. + * the next one. */ ctx.vbudget = vbudget; - current_hweight(iocg, NULL, &ctx.hw_inuse); __wake_up_locked_key(&iocg->waitq, TASK_NORMAL, &ctx); > > So, I'm not sure this is an improvement. It increases complication without > actually gaining anything. > > Thanks. >