Received: by 2002:a05:6358:11c7:b0:104:8066:f915 with SMTP id i7csp882217rwl; Fri, 31 Mar 2023 03:57:58 -0700 (PDT) X-Google-Smtp-Source: AKy350ZrW7G8lRA6CzcjebgADMFjKc4Ho/mzYYRj+LqM3d27bSmcPfHue7DTEzlRk5+CTPvimS8u X-Received: by 2002:a05:6a00:2d0b:b0:62d:b0ab:a05d with SMTP id fa11-20020a056a002d0b00b0062db0aba05dmr5805654pfb.0.1680260277983; Fri, 31 Mar 2023 03:57:57 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1680260277; cv=none; d=google.com; s=arc-20160816; b=xcigdOv6yOa+iuFZFOR+Jo/k1sP2X3oB6Qd9DOyom9e835FFRASMSHeEx6OXOcDulo dD/Y6nqlFRBG4iyMQ0N2N64ooozwbLMZAus3/YLNXhHcPyJTeK0N3fNNOHQC+jXPvjyr 8kGeYFtaM4ZGOYiDoqiid0EQYEx0jBnedT68YbhOq7k7lgvKL0+26nKxAdNPAy48q8u9 nNYnNO6UYwhRXP3KEZKBzcRlsnnq2NG6GSJgqt3DgaMg7UfQawKChCloMN4cHqPXyBxn FEysokkOJoQRRsxfzlmTjPlGjvYdZJHB7KmB4F8nV9lDGpJBNV6kdPzO2V0+aJ38F2ls 1H5w== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:in-reply-to:content-transfer-encoding :content-disposition:mime-version:references:message-id:subject:cc :to:date:from:dkim-signature; bh=PPD6LoSAh/kDy/eZUAGrxPbkpCHApPVs82PFiXVfopQ=; b=EcbSZh/yjN0TT/B/IeDXX9JWJI2KXolzNR8A/lMQn1ItYbzDFuP7GGaT4McjoNB8oU DGJqwqWI0vCZCgJ6tIyDLSr/GHcBJm0D/ZMISTbvc7FI+7Np7joHb5I1tuoEbqGlGAG4 EHoY+xKSb9m4+On06VSzbqFgOZlIWy8tA9wPnUiYIk2ATNDkFoPK3ji2JCXe8yKsXoaz qAYBclYtcF9otCW/LP/FsldPS18U6sCwQXhkj4kJSxPefcO2zVz6uxsNJQo+N2EGvh0+ BtmU3fduWPB1G8yUQB3dkPt1vbKH3kGZtWoGF7c2EKT+FpIhyj5CgXhcb+DBT2Oy31LZ uBrw== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@gmail.com header.s=20210112 header.b=RBVUOphL; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Return-Path: Received: from out1.vger.email (out1.vger.email. [2620:137:e000::1:20]) by mx.google.com with ESMTP id w62-20020a638241000000b0050308cba870si2134112pgd.137.2023.03.31.03.57.46; Fri, 31 Mar 2023 03:57:57 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20210112 header.b=RBVUOphL; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230039AbjCaKzq (ORCPT + 99 others); Fri, 31 Mar 2023 06:55:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58242 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230193AbjCaKzo (ORCPT ); Fri, 31 Mar 2023 06:55:44 -0400 Received: from mail-lj1-x22c.google.com (mail-lj1-x22c.google.com [IPv6:2a00:1450:4864:20::22c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 26D271DFB3; Fri, 31 Mar 2023 03:55:40 -0700 (PDT) Received: by mail-lj1-x22c.google.com with SMTP id q14so22564726ljm.11; Fri, 31 Mar 2023 03:55:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; t=1680260139; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:date:from:from:to :cc:subject:date:message-id:reply-to; bh=PPD6LoSAh/kDy/eZUAGrxPbkpCHApPVs82PFiXVfopQ=; b=RBVUOphLWDRWn2joVC4oPodbER+nSLz0Pt2JPXW+VG3y7KdRAY3yn38U6EjuAYK94y 6OfjT8ogxNlZeOcvQfGeaAEkQ+XT1RSqxhIlPJB2o5hvh7KTDgRejLoRBjXsUt+UqXPE FL1GVihyYSjtlvfTV2DBR31vGSFFWhaklcWXVUEq1IwzYxyqqgPVjw0UdyJN8HyqHxzR w2s84s3OLkrlC6YxNBB8GFAo4zEhwkiOWOkyS+TYTMgbK5wPM9isvWU3pxVqrC5L3H1e x8rhg/GduZ2wgtYyxN8sZqsf59rU0rfDwHgDuKyub49pEkUz11PPArgMxIV6VZMznMql ofOg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1680260139; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:date:from :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=PPD6LoSAh/kDy/eZUAGrxPbkpCHApPVs82PFiXVfopQ=; b=KupJPtJfB9XNfRO3M7g1jK5GqedSC/XqEcBOeXbNcpUtzT2UbialjSzkYffdxunRG8 o6TGfbQYBn8WI2QjPebXGTN83L9rgEXB9Xss2wtY3pvl+8tJMSLo7+j0iIaFFD4bozqt Lndymi0XZUVjmAqoWUy4XiOmw+Y+UiFwITOgIgsZB4v2RkgHhuLGXSez5Qa0SHNzyMdL YYo7IDWWSlJK8sQGTfTKAitsAK0qmPRfiFMjAcQeg5i6Dg/mx1rihO4IDrJiTJwMA4ab HkB9TVHcyxMk054tqtI7OacsNfpqzLSHXVb4f9U1WpCfCUGb4Fc63fNytUdIBHcwStLo TIYw== X-Gm-Message-State: AAQBX9dRTOcwyLFbFvJ9KCyVsAsftAiy45oZgbPWnfX5aSXtXYE6H+ER CsneTo3w3SkDP6BVPXRch0Y= X-Received: by 2002:a2e:82d7:0:b0:29b:d641:2ab9 with SMTP id n23-20020a2e82d7000000b0029bd6412ab9mr7988674ljh.16.1680260138865; Fri, 31 Mar 2023 03:55:38 -0700 (PDT) Received: from pc636 (host-90-233-209-50.mobileonline.telia.com. [90.233.209.50]) by smtp.gmail.com with ESMTPSA id n20-20020a2e8794000000b0029573844d03sm304310lji.109.2023.03.31.03.55.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Mar 2023 03:55:38 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Fri, 31 Mar 2023 12:55:36 +0200 To: "Paul E. McKenney" Cc: Uladzislau Rezki , Joel Fernandes , "Zhang, Qiang1" , "Zhuo, Qiuxu" , RCU , quic_neeraju@quicinc.com, Boqun Feng , LKML , Oleksiy Avramchenko , Steven Rostedt , Frederic Weisbecker Subject: Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time Message-ID: References: <2cd8f407-2b77-48b1-9f17-9aa8e4ce9c64@paulmck-laptop> <20230330150933.GB2114899@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Spam-Status: No, score=-0.2 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 30, 2023 at 02:16:36PM -0700, Paul E. McKenney wrote: > On Thu, Mar 30, 2023 at 09:18:44PM +0200, Uladzislau Rezki wrote: > > On Thu, Mar 30, 2023 at 11:58:41AM -0700, Paul E. McKenney wrote: > > > On Thu, Mar 30, 2023 at 05:43:15PM +0200, Uladzislau Rezki wrote: > > > > On Thu, Mar 30, 2023 at 03:09:33PM +0000, Joel Fernandes wrote: > > > > > On Tue, Mar 28, 2023 at 08:26:13AM -0700, Paul E. McKenney wrote: > > > > > > On Mon, Mar 27, 2023 at 10:29:31PM -0400, Joel Fernandes wrote: > > > > > > > Hello, > > > > > > > > > > > > > > > On Mar 27, 2023, at 9:06 PM, Paul E. McKenney wrote: > > > > > > > > > > > > > > > > On Mon, Mar 27, 2023 at 11:21:23AM +0000, Zhang, Qiang1 wrote: > > > > > > > >>>> From: Uladzislau Rezki (Sony) > > > > > > > >>>> Sent: Tuesday, March 21, 2023 6:28 PM > > > > > > > >>>> [...] > > > > > > > >>>> Subject: [PATCH 1/1] Reduce synchronize_rcu() waiting time > > > > > > > >>>> > > > > > > > >>>> A call to a synchronize_rcu() can be expensive from time point of view. > > > > > > > >>>> Different workloads can be affected by this especially the ones which use this > > > > > > > >>>> API in its time critical sections. > > > > > > > >>>> > > > > > > > >>> > > > > > > > >>> This is interesting and meaningful research. ;-) > > > > > > > >>> > > > > > > > >>>> For example in case of NOCB scenario the wakeme_after_rcu() callback > > > > > > > >>>> invocation depends on where in a nocb-list it is located. Below is an example > > > > > > > >>>> when it was the last out of ~3600 callbacks: > > > > > > > >>> > > > > > > > >> > > > > > > > >> > > > > > > > >> > > > > > > > >> Can it be implemented separately as follows? it seems that the code is simpler > > > > > > > >> (only personal opinion) ????. > > > > > > > >> > > > > > > > >> But I didn't test whether this reduce synchronize_rcu() waiting time > > > > > > > >> > > > > > > > >> +static void rcu_poll_wait_gp(struct rcu_tasks *rtp) > > > > > > > >> +{ > > > > > > > >> + unsigned long gp_snap; > > > > > > > >> + > > > > > > > >> + gp_snap = start_poll_synchronize_rcu(); > > > > > > > >> + while (!poll_state_synchronize_rcu(gp_snap)) > > > > > > > >> + schedule_timeout_idle(1); > > > > > > > > > > > > > > > > I could be wrong, but my guess is that the guys working with > > > > > > > > battery-powered devices are not going to be very happy with this loop. > > > > > > > > > > > > > > > > All those wakeups by all tasks waiting for a grace period end up > > > > > > > > consuming a surprisingly large amount of energy. > > > > > > > > > > > > > > Is that really the common case? On the general topic of wake-ups: > > > > > > > Most of the time there should be only one > > > > > > > task waiting synchronously on a GP to end. If that is > > > > > > > true, then it feels like waking > > > > > > > up nocb Kthreads which indirectly wake other threads is doing more work than usual? > > > > > > > > > > > > A good question, and the number of outstanding synchronize_rcu() > > > > > > calls will of course be limited by the number of tasks in the system. > > > > > > But I myself have raised the ire of battery-powered embedded folks with > > > > > > a rather small number of wakeups, so... > > > > > > > > > > But unless I am missing something, even if there is single synchronize_rcu(), > > > > > you have a flurry of potential wakeups right now, instead of the bare minimum > > > > > I think. I have not measured how many wake ups, but I'd love to when I get > > > > > time. Maybe Vlad has some numbers. > > > > > > > > > I will measure and have a look at wake-ups. But, what we have for now is > > > > if there are two callers of synchronize_rcu() on different CPUs, i guess > > > > two nocb-kthreads have to handle it, thus two nocb-kthreads have to be > > > > awaken to do the work. This patch needs only one wake-up to serve all > > > > users. > > > > > > One wakeup per synchronize_rcu(), right? > > > > > The gp-kthread wake-ups only one work, in its turn a worker wake-ups all > > registered users of synchronize_rcu() for which a gp was passed. How many > > users of synchonize_rcu() awaken by one worker depends on how many were > > registered before initiating a new GP by the gp-kthread. > > > > > > Anyway, i will provide some data and analysis of it. > > > > > > Looking forward to seeing it! > > > > > Good. I will switch fully on it soon. I need to sort out some perf. > > issues at work. > > And if you are looking for reduced wakeups instead of lower latency for > synchronize_rcu(), I could see where the extra workqueue wakeup might > be a problem for you. > > Assuming that this is all default-off, you could keep a count of the > number of required wakeups for each grace period (indexed as usual by > the bottom few bits of the grace-period counter without the low-order > state bits), and do the wakeups directly from the grace-period kthread > if there are not all that many of them. > At least if there is only one user of synchronize_rcu(), we can wake it directly, i mean to invoke comlete() from the gp-kthread. I think we should split such parts into different patches. > > Except that, given that workqueues try hard to make the handler be on the > same CPU as the one that did the corresponding schedule_work() invocation, > it is not clear that this particular wakeup is really costing you enough > to notice. (That CPU is not idle, after all.) But there is nothing > quite like measuring the actual energy consumption on real hardware! > AFAICR the schedule_work() wants to use current CPU, indeed. For few users it might be OK to comlete() them directly. Energy wise, +1 wake-up of our worker to kick all users, if not "direct" option. I do not think energy is a problem here. -- Uladzislau Rezki