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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B6D0CC433EF for ; Wed, 22 Jun 2022 11:46:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1357731AbiFVLqt (ORCPT ); Wed, 22 Jun 2022 07:46:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57056 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355742AbiFVLqr (ORCPT ); Wed, 22 Jun 2022 07:46:47 -0400 Received: from alexa-out-sd-01.qualcomm.com (alexa-out-sd-01.qualcomm.com [199.106.114.38]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 06B1E33A23; Wed, 22 Jun 2022 04:46:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=quicinc.com; i=@quicinc.com; q=dns/txt; s=qcdkim; t=1655898406; x=1687434406; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=PLePtGK/nNadtMrdup55F1Wn755t1dKBv+FuXP1GQkU=; b=GWizppB8KtezXisVGMan2u3AF3nfi6qc8eW4/6Nblq5Dur8u7c/DcF6C Oc6uwviUvHiLJNtJISaLWEvc2PevYt7vJVM/g86Bxql55R9ZsaiSeoeah pCYgwzLVO+5tsZsuZTMN1ZoG/AAxuf8R81Z28yYulfKSDO2NxLXsiLc+p Y=; Received: from unknown (HELO ironmsg04-sd.qualcomm.com) ([10.53.140.144]) by alexa-out-sd-01.qualcomm.com with ESMTP; 22 Jun 2022 04:46:45 -0700 X-QCInternal: smtphost Received: from nasanex01b.na.qualcomm.com ([10.46.141.250]) by ironmsg04-sd.qualcomm.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Jun 2022 04:46:45 -0700 Received: from [10.50.44.13] (10.80.80.8) by nasanex01b.na.qualcomm.com (10.46.141.250) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.986.22; Wed, 22 Jun 2022 04:46:43 -0700 Message-ID: <65e9c3bf-af54-3726-dff5-208a70473f65@quicinc.com> Date: Wed, 22 Jun 2022 17:16:39 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH rcu 01/12] rcu: Decrease FQS scan wait time in case of callback overloading Content-Language: en-US To: CC: , , , References: <20220620222022.GA3839466@paulmck-ThinkPad-P17-Gen-1> <20220620222032.3839547-1-paulmck@kernel.org> <87c17e9a-565a-d717-3534-83a4c506b984@quicinc.com> <20220621221925.GQ1790663@paulmck-ThinkPad-P17-Gen-1> From: Neeraj Upadhyay In-Reply-To: <20220621221925.GQ1790663@paulmck-ThinkPad-P17-Gen-1> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.80.80.8] X-ClientProxiedBy: nasanex01b.na.qualcomm.com (10.46.141.250) To nasanex01b.na.qualcomm.com (10.46.141.250) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/22/2022 3:49 AM, Paul E. McKenney wrote: > On Tue, Jun 21, 2022 at 10:59:58AM +0530, Neeraj Upadhyay wrote: >> >> >> On 6/21/2022 3:50 AM, Paul E. McKenney wrote: >>> The force-quiesce-state loop function rcu_gp_fqs_loop() checks for >>> callback overloading and does an immediate initial scan for idle CPUs >>> if so. However, subsequent rescans will be carried out at as leisurely a >>> rate as they always are, as specified by the rcutree.jiffies_till_next_fqs >>> module parameter. It might be tempting to just continue immediately >>> rescanning, but this turns the RCU grace-period kthread into a CPU hog. >>> It might also be tempting to reduce the time between rescans to a single >>> jiffy, but this can be problematic on larger systems. >>> >>> This commit therefore divides the normal time between rescans by three, >>> rounding up. Thus a small system running at HZ=1000 that is suffering >>> from callback overload will wait only one jiffy instead of the normal >>> three between rescans. >>> >>> Signed-off-by: Paul E. McKenney >>> --- >>> kernel/rcu/tree.c | 5 +++++ >>> 1 file changed, 5 insertions(+) >>> >>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c >>> index c25ba442044a6..c19d5926886fb 100644 >>> --- a/kernel/rcu/tree.c >>> +++ b/kernel/rcu/tree.c >>> @@ -1993,6 +1993,11 @@ static noinline_for_stack void rcu_gp_fqs_loop(void) >>> WRITE_ONCE(rcu_state.jiffies_kick_kthreads, >>> jiffies + (j ? 3 * j : 2)); >>> } >>> + if (rcu_state.cbovld) { >>> + j = (j + 2) / 3; >>> + if (j <= 0) >>> + j = 1; >>> + } >> >> We update 'j' here, after setting rcu_state.jiffies_force_qs >> >> WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j) >> >> So, we return from swait_event_idle_timeout_exclusive after 1/3 time >> duration. >> >> swait_event_idle_timeout_exclusive(rcu_state.gp_wq, >> rcu_gp_fqs_check_wake(&gf), j); >> >> This can result in !timer_after check to return false and we will >> enter the 'else' (stray signal block) code? >> >> This might not matter for first 2 fqs loop iterations, where >> RCU_GP_FLAG_OVLD is set in 'gf', but subsequent iterations won't benefit >> from this patch? >> >> >> if (!time_after(rcu_state.jiffies_force_qs, jiffies) || >> (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) { >> ... >> } else { >> /* Deal with stray signal. */ >> } >> >> >> So, do we need to move this calculation above the 'if' block which sets >> rcu_state.jiffies_force_qs? >> if (!ret) { >> >> WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + >> j);... >> } > > Good catch, thank you! How about the updated patch shown below? > Looks good to me. Thanks Neeraj > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 77de092c78f549b5c28075bfee9998a525d21f84 > Author: Paul E. McKenney > Date: Tue Apr 12 15:08:14 2022 -0700 > > rcu: Decrease FQS scan wait time in case of callback overloading > > The force-quiesce-state loop function rcu_gp_fqs_loop() checks for > callback overloading and does an immediate initial scan for idle CPUs > if so. However, subsequent rescans will be carried out at as leisurely a > rate as they always are, as specified by the rcutree.jiffies_till_next_fqs > module parameter. It might be tempting to just continue immediately > rescanning, but this turns the RCU grace-period kthread into a CPU hog. > It might also be tempting to reduce the time between rescans to a single > jiffy, but this can be problematic on larger systems. > > This commit therefore divides the normal time between rescans by three, > rounding up. Thus a small system running at HZ=1000 that is suffering > from callback overload will wait only one jiffy instead of the normal > three between rescans. > > [ paulmck: Apply Neeraj Upadhyay feedback. ] > > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index c25ba442044a6..52094e72866e5 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1983,7 +1983,12 @@ static noinline_for_stack void rcu_gp_fqs_loop(void) > gf = RCU_GP_FLAG_OVLD; > ret = 0; > for (;;) { > - if (!ret) { > + if (rcu_state.cbovld) { > + j = (j + 2) / 3; > + if (j <= 0) > + j = 1; > + } > + if (!ret || time_before(jiffies + j, rcu_state.jiffies_force_qs)) { > WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j); > /* > * jiffies_force_qs before RCU_GP_WAIT_FQS state