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 22986C433EF for ; Tue, 21 Jun 2022 05:14:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344915AbiFUFOJ (ORCPT ); Tue, 21 Jun 2022 01:14:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39102 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232935AbiFUFOC (ORCPT ); Tue, 21 Jun 2022 01:14:02 -0400 Received: from alexa-out.qualcomm.com (alexa-out.qualcomm.com [129.46.98.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7C99015808; Mon, 20 Jun 2022 22:14:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=quicinc.com; i=@quicinc.com; q=dns/txt; s=qcdkim; t=1655788440; x=1687324440; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=zD3t7P3beJnqprKw6pK4SaA1mOaXHFNana/obLJCFKc=; b=uLtzbaLCcmxgKNUJ6vUmdk0MhnsXSmrH4BVg+5zp9LL/z0Q21TtmZ4hH DhHzZcJ+HCVuasQqbbzPubc69rTXMlglOTf0ZUiOjBt92t3fWv9BAcgQ2 yxntmRN4foc64h8rNXajiXu+ED2hIuRx1fdrSe7VXjyxZ+RjD8CKoVXPx s=; Received: from ironmsg02-sd.qualcomm.com ([10.53.140.142]) by alexa-out.qualcomm.com with ESMTP; 20 Jun 2022 22:14:00 -0700 X-QCInternal: smtphost Received: from nasanex01b.na.qualcomm.com ([10.46.141.250]) by ironmsg02-sd.qualcomm.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Jun 2022 22:11:59 -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; Mon, 20 Jun 2022 22:11:57 -0700 Message-ID: <3988b203-0a65-526e-b445-c2ee9ea0f4e8@quicinc.com> Date: Tue, 21 Jun 2022 10:41:54 +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 3/3] doc: Document rcutree.nocb_nobypass_lim_per_jiffy kernel parameter Content-Language: en-US To: "Paul E. McKenney" , CC: , , , Frederic Weisbecker , Uladzislau Rezki References: <20220620221733.GA3839136@paulmck-ThinkPad-P17-Gen-1> <20220620221757.3839212-3-paulmck@kernel.org> From: Neeraj Upadhyay In-Reply-To: <20220620221757.3839212-3-paulmck@kernel.org> 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 6/21/2022 3:47 AM, Paul E. McKenney wrote: > This commit provides documentation for the kernel parameter controlling > RCU's handling of callback floods on offloaded (rcu_nocbs) CPUs. > This parameter might be obscure, but it is always there when you need it. > > Reported-by: Frederic Weisbecker > Reported-by: Uladzislau Rezki > Signed-off-by: Paul E. McKenney > --- Reviewed-by: Neeraj Upadhyay > Documentation/admin-guide/kernel-parameters.txt | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index bdf431bdbfdc4..a30890141b1a5 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -4681,6 +4681,19 @@ > invoked in the context of an rcuoc kthread, which > scheduler will preempt as it does any other task. > > + rcutree.nocb_nobypass_lim_per_jiffy= [KNL] > + On callback-offloaded (rcu_nocbs) CPUs, > + RCU reduces the lock contention that would > + otherwise be caused callback floods through Minor : "caused by" ? Thanks Neeraj > + use of the ->nocb_bypass list. However, in the > + common non-flooded case, RCU queues directly to > + the main ->cblist in order to avoid the extra > + overhead of the ->nocb_bypass list and its lock. > + But if there are too many callbacks queued during > + a single jiffy, RCU pre-queues the callbacks into > + the ->nocb_bypass queue. The definition of "too > + many" is supplied by this kernel boot parameter. > + > rcutree.rcu_nocb_gp_stride= [KNL] > Set the number of NOCB callback kthreads in > each group, which defaults to the square root