Received: by 2002:a05:6358:11c7:b0:104:8066:f915 with SMTP id i7csp2276679rwl; Thu, 30 Mar 2023 08:14:28 -0700 (PDT) X-Google-Smtp-Source: AKy350bkVzoY5UF8Xl7kjItnlCOon0TjmA0i4Y5WH7YFOJithEs/x0ERwyVIWGyJKgPL7Uy5E9u1 X-Received: by 2002:a17:906:5307:b0:933:2e79:4632 with SMTP id h7-20020a170906530700b009332e794632mr18966743ejo.1.1680189267966; Thu, 30 Mar 2023 08:14:27 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1680189267; cv=none; d=google.com; s=arc-20160816; b=cOYy/UHFZxmhiuNbeu8h6T5Uin6mRq4GS47UNX6OMM/dcE1Sm/Qu8aFOUzNtwynvTv kY5f3qyMaXAMM6kYha3n0HoSGdW8Mp5X+PuZ4pMA0ahEkoXvq3XYv/ZFUiedgymoe3qx SEnzIIP7wl10lDEbAGKYkPWnxdoQ+Vdm0vsz60r2JAucxe+SY7FEHy0n7Yr2vSdvkcq4 FbqL6FncfZJhIRkt9zJkZ/sGYkwq4eHoofm2q9j0i58+9sxoFymB28ddCXFS4v669itq lgKzPvARDhu0c1dfm/n/vMoogSbIsmeMEn7SP3AzVIF+RP9eC7GA8qDD643ePP8OkPkC BESA== 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:from:date:dkim-signature; bh=d9SSEYK9+rLxf8rsg8ZhJkHKzQe51MzIdNwTny7GCDE=; b=d3JsydRyIvql2+YY3Fb43N8IZDxbH76lGNOdChmeh6ECIoiwSpX8hQoSaAvsBxyJpo S3vOcRc5goMm6MMBWR4Oi4E0zKfm2yBIc1dsSB3RvNt12kPrfKwpN1BmWq3FM11fHjQt t+eaIigqe9drP/aDHOKdd8FF2DZ0OBtwrk3weNN2p3tNRguijrc/diOYFqXTjZGIR4Xj kEbV6VZBR8H+VpuO3BMEyM2iN7nrr7yKkOI36s3JA3xXFaeS6Po/ARXICHrK+FvY8lX+ TWX5rkQVtXKGF0t/0vkOWqnDl5Le6zYDFPsVgLhWkVoAg7NSEfoQgzX3dvo1Y4Xbzzpb T4IA== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@joelfernandes.org header.s=google header.b=m6UVd1hd; 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 Return-Path: Received: from out1.vger.email (out1.vger.email. [2620:137:e000::1:20]) by mx.google.com with ESMTP id z18-20020a170906271200b00930c37d7919si1088260ejc.673.2023.03.30.08.13.17; Thu, 30 Mar 2023 08:14:27 -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=@joelfernandes.org header.s=google header.b=m6UVd1hd; 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 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232796AbjC3PK7 (ORCPT + 99 others); Thu, 30 Mar 2023 11:10:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57162 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231461AbjC3PK6 (ORCPT ); Thu, 30 Mar 2023 11:10:58 -0400 Received: from mail-qt1-x835.google.com (mail-qt1-x835.google.com [IPv6:2607:f8b0:4864:20::835]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E20583C3D for ; Thu, 30 Mar 2023 08:09:36 -0700 (PDT) Received: by mail-qt1-x835.google.com with SMTP id t19so18720132qta.12 for ; Thu, 30 Mar 2023 08:09:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; t=1680188974; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:from:to :cc:subject:date:message-id:reply-to; bh=d9SSEYK9+rLxf8rsg8ZhJkHKzQe51MzIdNwTny7GCDE=; b=m6UVd1hdrKFSzgvADYkcehDr1RbP+WDP6ki0+MRYiZePeMzYfH5bnL8iaZpk1yobQ9 Ij9sX5WPve0A+sxBY7mx8QBV6b9MwsVb7Ppm9uYSBSUpSAmJQ5/9d+Eftrlv2kmvdugk hYP6HwviiUDgkgvncrcG1qSBEZVp5Gp72jLwg= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1680188974; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=d9SSEYK9+rLxf8rsg8ZhJkHKzQe51MzIdNwTny7GCDE=; b=yD6DX3sjMI6ZfI9tA8zyeYMAhSwkncNPSZmoqs0M4lJ6nUoAxiAHLnH6Zg5c/WSuPX C1lOoydjTEhR6HXHM/ruvAoh70EsQIYhtpkqTPkN0dz1eLbeqUfxIeeT3gCkgUxHuXwc qvF9axxJL+s79fvgeJ6vepccaX4cgSdKBd2IFgOyxyaWUXZuqZvVmu+nnby2RHtdNn+a 73sejBOk/Ie5vPjWh64/xBMD7ykDgH2ypktirxJBnIr6i5gVufFZXFJg2AjKwaa3xYzX Ln2Cs6glaMndEkFPD4nzfN3Umu/X0mS0xvZxzE1kYvUrhG90Itle/VHjJI38cqrsX4TP idaA== X-Gm-Message-State: AAQBX9ff5RyQr0hhNKn5K42IluYug4jbeprqAcn4m1HrPYuFLRh98ATI Upa3L4OPBtBtvAozoazrXnwj6A== X-Received: by 2002:a05:622a:190f:b0:3bf:b5fe:372d with SMTP id w15-20020a05622a190f00b003bfb5fe372dmr37528110qtc.61.1680188974011; Thu, 30 Mar 2023 08:09:34 -0700 (PDT) Received: from localhost (129.239.188.35.bc.googleusercontent.com. [35.188.239.129]) by smtp.gmail.com with ESMTPSA id d3-20020ac847c3000000b003e4f1b3ce43sm2124925qtr.50.2023.03.30.08.09.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Mar 2023 08:09:33 -0700 (PDT) Date: Thu, 30 Mar 2023 15:09:33 +0000 From: Joel Fernandes To: "Paul E. McKenney" Cc: "Zhang, Qiang1" , Uladzislau Rezki , "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: <20230330150933.GB2114899@google.com> References: <2cd8f407-2b77-48b1-9f17-9aa8e4ce9c64@paulmck-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2cd8f407-2b77-48b1-9f17-9aa8e4ce9c64@paulmck-laptop> X-Spam-Status: No, score=-0.2 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,DKIM_VALID_EF,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 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. > And on larger systems there can be a tradeoff between contention on > the one hand and number of wakeups on the other. > > The original nocb implementation in fact had the grace-period kthead > waking up all of what are now called rcuoc kthreads. The indirect scheme > reduced the total number of wakeups by up to 50% and also reduced the > CPU consumption of the grace-period kthread, which otherwise would have > become a bottleneck on large systems. Thanks for the background. > And also, a scheme that directly wakes tasks waiting in synchronize_rcu() > might well use the same ->nocb_gp_wq[] waitqueues that are used by the > rcuog kthreads, if that is what you were getting at. Yes that's what I was getting at. I thought Vlad was going for doing direct wake ups from the main RCU GP thread that orchestates RCU grace period cycles. > > I am curious to measure how much does Vlad patch reduce wakeups in the common case. > > Sounds like a good thing to measure! Ok. At the moment I am preparing 2 talks I am giving at OSPM for real-time and timers. Plus preparing the PR, so I'm fully booked. :( [and the LWN article..]. > > > I was also wondering how Vlad patch effects RCU-barrier ordering. I guess > > we want the wake up to happen in the order of > > other callbacks also waiting. > > OK, I will bite. Why would rcu_barrier() need to care about the > synchronize_rcu() invocations if they no longer used call_rcu()? Hm, I was just going for the fact that it is a behavioral change. Not illuding that it would certainly cause an issue. As we know, Linux kernel developers have interesting ways of using RCU APIs. :-) But yes, it may not be an issue considering expedited synchronize_rcu() also has such behavior anyway, if I'm not mistaken. > > One last note, most battery powered systems are perhaps already using expedited RCU ;-) > > Good point. And that does raise the question of exactly what workloads > and systems want faster wakeups from synchronize_rcu() and cannot get > this effect from expedited grace periods. Maybe the kind of workloads that don't need GP completion very quickly, but just want to reduce wakeups. The wakeups do have a cost, the scheduler can wake up several idle CPUs to "spread the awakened load" and cause wastage power. And also contend on locks during the wake up. thanks, - Joel > > - Joel > > > > > > > > Thanx, Paul > > > > > >> +} > > >> + > > >> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func); > > >> +DEFINE_RCU_TASKS(rcu_poll, rcu_poll_wait_gp, call_rcu_poll, > > >> + "RCU Poll"); > > >> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func) > > >> +{ > > >> + call_rcu_tasks_generic(rhp, func, &rcu_poll); > > >> +} > > >> +EXPORT_SYMBOL_GPL(call_rcu_poll); > > >> + > > >> +void synchronize_rcu_poll(void) > > >> +{ > > >> + synchronize_rcu_tasks_generic(&rcu_poll); > > >> +} > > >> +EXPORT_SYMBOL_GPL(synchronize_rcu_poll); > > >> + > > >> +static int __init rcu_spawn_poll_kthread(void) > > >> +{ > > >> + cblist_init_generic(&rcu_poll); > > >> + rcu_poll.gp_sleep = HZ / 10; > > >> + rcu_spawn_tasks_kthread_generic(&rcu_poll); > > >> + return 0; > > >> +} > > >> > > >> Thanks > > >> Zqiang > > >> > > >> > > >>>> > > >>>> > > >>>> <...>-29 [001] d..1. 21950.145313: rcu_batch_start: rcu_preempt > > >>>> CBs=3613 bl=28 > > >>>> ... > > >>>> <...>-29 [001] ..... 21950.152578: rcu_invoke_callback: rcu_preempt > > >>>> rhp=00000000b2d6dee8 func=__free_vm_area_struct.cfi_jt > > >>>> <...>-29 [001] ..... 21950.152579: rcu_invoke_callback: rcu_preempt > > >>>> rhp=00000000a446f607 func=__free_vm_area_struct.cfi_jt > > >>>> <...>-29 [001] ..... 21950.152580: rcu_invoke_callback: rcu_preempt > > >>>> rhp=00000000a5cab03b func=__free_vm_area_struct.cfi_jt > > >>>> <...>-29 [001] ..... 21950.152581: rcu_invoke_callback: rcu_preempt > > >>>> rhp=0000000013b7e5ee func=__free_vm_area_struct.cfi_jt > > >>>> <...>-29 [001] ..... 21950.152582: rcu_invoke_callback: rcu_preempt > > >>>> rhp=000000000a8ca6f9 func=__free_vm_area_struct.cfi_jt > > >>>> <...>-29 [001] ..... 21950.152583: rcu_invoke_callback: rcu_preempt > > >>>> rhp=000000008f162ca8 func=wakeme_after_rcu.cfi_jt > > >>>> <...>-29 [001] d..1. 21950.152625: rcu_batch_end: rcu_preempt CBs- > > >>>> invoked=3612 idle=.... > > >>>> > > >>>> > > >>> > > >>> Did the results above tell us that CBs-invoked=3612 during the time 21950.145313 ~ 21950.152625? > > >>> > > >>> Yes. > > >>> > > >>> > > >>> If possible, may I know the steps, commands, and related parameters to produce the results above? > > >>> Thank you! > > >>> > > >>> Build the kernel with CONFIG_RCU_TRACE configuration. Update your "set_event" > > >>> file with appropriate traces: > > >>> > > >>> > > >>> XQ-DQ54:/sys/kernel/tracing # echo rcu:rcu_batch_start rcu:rcu_batch_end rcu:rcu_invoke_callback > set_event > > >>> > > >>> XQ-DQ54:/sys/kernel/tracing # cat set_event > > >>> rcu:rcu_batch_start > > >>> rcu:rcu_invoke_callback > > >>> rcu:rcu_batch_end > > >>> XQ-DQ54:/sys/kernel/tracing # > > >>> > > >>> > > >>> Collect traces as much as you want: XQ-DQ54:/sys/kernel/tracing # echo 1 > tracing_on; sleep 10; echo 0 > tracing_on > > >>> Next problem is how to parse it. Of course you will not be able to parse > > >>> megabytes of traces. For that purpose i use a special C trace parser. > > >>> If you need an example please let me know i can show here. > > >>> > > >>> -- > > >>> Uladzislau Rezki