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[2620:137:e000::1:20]) by mx.google.com with ESMTP id k70-20020a638449000000b0051323712244si9124397pgd.2.2023.03.27.19.31.43; Mon, 27 Mar 2023 19:31:55 -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=Mh2Pg+vn; 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 S229897AbjC1Cap (ORCPT + 99 others); Mon, 27 Mar 2023 22:30:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60734 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229878AbjC1Can (ORCPT ); Mon, 27 Mar 2023 22:30:43 -0400 Received: from mail-qv1-xf2e.google.com (mail-qv1-xf2e.google.com [IPv6:2607:f8b0:4864:20::f2e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8A3AD30ED for ; Mon, 27 Mar 2023 19:30:08 -0700 (PDT) Received: by mail-qv1-xf2e.google.com with SMTP id g9so8286832qvt.8 for ; Mon, 27 Mar 2023 19:30:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; t=1679970583; h=to:references:message-id:date:cc:in-reply-to:from:mime-version :subject:content-transfer-encoding:from:to:cc:subject:date :message-id:reply-to; bh=pWz6d1QehWZ2JoCvpbDcTQFIKEH+s3VBG9kI0UG6rOM=; b=Mh2Pg+vnSUCd/9ByY5vk8n7ODXG6gL+h3nDPbFBtMK4pOAoKg5V74LbuB/KmZamkzk pzxNrePZxr1qCVPEi6bzVbSFtUKZihmAIgCzVI1CYiQi5V09x/WAV/4yLnXK95eg/4xi AfG5ttrBmbNG1kFJSX96AFGG9hzDC9FoZ6MAc= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1679970583; h=to:references:message-id:date:cc:in-reply-to:from:mime-version :subject:content-transfer-encoding:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=pWz6d1QehWZ2JoCvpbDcTQFIKEH+s3VBG9kI0UG6rOM=; b=D/xJeVBBzkCY2rrKoIMyjdb+QKr+zUg9B2oQHOyZFjBED7LKK5MZm73ajdfJlpIRKe E2FUu/Rv/ZAr6EAi3pnN27xnUx79j4ghUXBybJ8n4hd8XUX0qGiUpJGHeDkNQeGh4j43 hnlq1FmNtJja8cgDu9yLMvG8t/ME11nK7q80y0jpbjqF0/Ms48GLYddprmaSx4ihlqnE JKiJJpQDYv7vqTGlgAtrefcDMAy/WCojS5CYS1QgydTNQax8kf4qUkk5fnXMDKpfK0YL VM47taLPTQccMBHUbKMORHkR2OU3uJBE3ksk3gDlnsF/NBRa1eWnAxeuKk2ZqGAdEPBa GNVw== X-Gm-Message-State: AAQBX9fhCsCGDwVeujH/v4r3xZB2FqwuJ2WoN0BujZhadjMqSINK4Jhj QUh2f9SMgRiQJg9oo4pJr5ZJVA== X-Received: by 2002:a05:6214:20e8:b0:571:13c:6806 with SMTP id 8-20020a05621420e800b00571013c6806mr22424370qvk.33.1679970583479; Mon, 27 Mar 2023 19:29:43 -0700 (PDT) Received: from smtpclient.apple ([2600:1003:b130:c6c6:6401:1533:c2de:a27d]) by smtp.gmail.com with ESMTPSA id x14-20020a0cfe0e000000b005dd8b9345e3sm3423380qvr.123.2023.03.27.19.29.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Mar 2023 19:29:42 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time Mime-Version: 1.0 (1.0) X-Apple-Notify-Thread: NO X-Universally-Unique-Identifier: F8192FEA-755F-4711-984A-533C5ADE2112 From: Joel Fernandes In-Reply-To: Cc: "Zhang, Qiang1" , Uladzislau Rezki , "Zhuo, Qiuxu" , RCU , quic_neeraju@quicinc.com, Boqun Feng , LKML , Oleksiy Avramchenko , Steven Rostedt , Frederic Weisbecker Date: Mon, 27 Mar 2023 22:29:31 -0400 X-Apple-Message-Smime-Encrypt: NO Message-Id: References: To: paulmck@kernel.org X-Mailer: iPhone Mail (20B101) X-Spam-Status: No, score=1.5 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS, URIBL_BLACK autolearn=no autolearn_force=no version=3.4.6 X-Spam-Level: * 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 Hello, > On Mar 27, 2023, at 9:06 PM, Paul E. McKenney wrote: >=20 > =EF=BB=BFOn 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 >>>>=20 >>>> 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 u= se this >>>> API in its time critical sections. >>>>=20 >>>=20 >>> This is interesting and meaningful research. ;-) >>>=20 >>>> 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 e= xample >>>> when it was the last out of ~3600 callbacks: >>>=20 >>=20 >>=20 >>=20 >> Can it be implemented separately as follows? it seems that the code is s= impler >> (only personal opinion) =F0=9F=98=8A. >>=20 >> But I didn't test whether this reduce synchronize_rcu() waiting time >>=20 >> +static void rcu_poll_wait_gp(struct rcu_tasks *rtp) >> +{ >> + unsigned long gp_snap; >> + >> + gp_snap =3D start_poll_synchronize_rcu(); >> + while (!poll_state_synchronize_rcu(gp_snap)) >> + schedule_timeout_idle(1); >=20 > 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. >=20 > 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? I am curious to measure how much does Vlad patch reduce wakeups in the commo= n case. 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. One last note, most battery powered systems are perhaps already using expedi= ted RCU ;-) Thoughts? - Joel=20 >=20 > Thanx, Paul >=20 >> +} >> + >> +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 =3D HZ / 10; >> + rcu_spawn_tasks_kthread_generic(&rcu_poll); >> + return 0; >> +} >>=20 >> Thanks >> Zqiang >>=20 >>=20 >>>>=20 >>>> >>>> <...>-29 [001] d..1. 21950.145313: rcu_batch_start: rcu_preempt >>>> CBs=3D3613 bl=3D28 >>>> ... >>>> <...>-29 [001] ..... 21950.152578: rcu_invoke_callback: rcu_preem= pt >>>> rhp=3D00000000b2d6dee8 func=3D__free_vm_area_struct.cfi_jt >>>> <...>-29 [001] ..... 21950.152579: rcu_invoke_callback: rcu_preem= pt >>>> rhp=3D00000000a446f607 func=3D__free_vm_area_struct.cfi_jt >>>> <...>-29 [001] ..... 21950.152580: rcu_invoke_callback: rcu_preem= pt >>>> rhp=3D00000000a5cab03b func=3D__free_vm_area_struct.cfi_jt >>>> <...>-29 [001] ..... 21950.152581: rcu_invoke_callback: rcu_preem= pt >>>> rhp=3D0000000013b7e5ee func=3D__free_vm_area_struct.cfi_jt >>>> <...>-29 [001] ..... 21950.152582: rcu_invoke_callback: rcu_preem= pt >>>> rhp=3D000000000a8ca6f9 func=3D__free_vm_area_struct.cfi_jt >>>> <...>-29 [001] ..... 21950.152583: rcu_invoke_callback: rcu_preem= pt >>>> rhp=3D000000008f162ca8 func=3Dwakeme_after_rcu.cfi_jt >>>> <...>-29 [001] d..1. 21950.152625: rcu_batch_end: rcu_preempt CBs= - >>>> invoked=3D3612 idle=3D.... >>>> >>>>=20 >>>=20 >>> Did the results above tell us that CBs-invoked=3D3612 during the time 21= 950.145313 ~ 21950.152625? >>>=20 >>> Yes. >>>=20 >>>=20 >>> If possible, may I know the steps, commands, and related parameters to p= roduce the results above? >>> Thank you! >>>=20 >>> Build the kernel with CONFIG_RCU_TRACE configuration. Update your "set_e= vent" >>> file with appropriate traces: >>>=20 >>> >>> XQ-DQ54:/sys/kernel/tracing # echo rcu:rcu_batch_start rcu:rcu_batch_end= rcu:rcu_invoke_callback > set_event >>>=20 >>> 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 # >>> >>>=20 >>> 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. >>>=20 >>> -- >>> Uladzislau Rezki