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 X-Spam-Level: X-Spam-Status: No, score=-8.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 73625C433DF for ; Tue, 23 Jun 2020 06:19:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4692E20774 for ; Tue, 23 Jun 2020 06:19:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=mg.codeaurora.org header.i=@mg.codeaurora.org header.b="vUCJgvja" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730694AbgFWGTr (ORCPT ); Tue, 23 Jun 2020 02:19:47 -0400 Received: from mail29.static.mailgun.info ([104.130.122.29]:24907 "EHLO mail29.static.mailgun.info" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730510AbgFWGTq (ORCPT ); Tue, 23 Jun 2020 02:19:46 -0400 DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=mg.codeaurora.org; q=dns/txt; s=smtp; t=1592893185; h=Content-Transfer-Encoding: Content-Type: In-Reply-To: MIME-Version: Date: Message-ID: From: References: Cc: To: Subject: Sender; bh=qSIBjsymwcejYBr9cEeko6qfS3ROa3wXIu8BjipLX44=; b=vUCJgvjahzg/cfLAr9v0fpaYWcnWL3G0ASF12fejCMEvTtcO5lZeGUTxB74P5s4N5MnwdHIl zKpECpu9vuFWTivfuOsjOcLDFwWuwf1724xi2fTiL/NWzZQvIlSsRp3qQbYOZG3faN/rN4y1 hV4ZyMcesyh1UJY0mHisNY062tI= X-Mailgun-Sending-Ip: 104.130.122.29 X-Mailgun-Sid: WyI0MWYwYSIsICJsaW51eC1rZXJuZWxAdmdlci5rZXJuZWwub3JnIiwgImJlOWU0YSJd Received: from smtp.codeaurora.org (ec2-35-166-182-171.us-west-2.compute.amazonaws.com [35.166.182.171]) by smtp-out-n01.prod.us-east-1.postgun.com with SMTP id 5ef19f00a6e154319f7bb3cc (version=TLS1.2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256); Tue, 23 Jun 2020 06:19:44 GMT Received: by smtp.codeaurora.org (Postfix, from userid 1001) id 2D9CDC433A0; Tue, 23 Jun 2020 06:19:44 +0000 (UTC) Received: from [192.168.0.100] (unknown [124.123.165.228]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) (Authenticated sender: neeraju) by smtp.codeaurora.org (Postfix) with ESMTPSA id EC7CFC433C8; Tue, 23 Jun 2020 06:19:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org EC7CFC433C8 Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; spf=none smtp.mailfrom=neeraju@codeaurora.org Subject: Re: [PATCH] rcu/tree: Force quiescent state on callback overload To: paulmck@kernel.org Cc: josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org References: <1592764647-2452-1-git-send-email-neeraju@codeaurora.org> <20200621195052.GF9247@paulmck-ThinkPad-P72> <94686c2e-b589-2598-e658-42f13cec1216@codeaurora.org> <20200622031325.GG9247@paulmck-ThinkPad-P72> <20200622225356.GT9247@paulmck-ThinkPad-P72> From: Neeraj Upadhyay Message-ID: Date: Tue, 23 Jun 2020 11:49:38 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200622225356.GT9247@paulmck-ThinkPad-P72> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Paul, On 6/23/2020 4:23 AM, Paul E. McKenney wrote: > On Mon, Jun 22, 2020 at 09:16:24AM +0530, Neeraj Upadhyay wrote: >> Hi Paul, >> >> On 6/22/2020 8:43 AM, Paul E. McKenney wrote: >>> On Mon, Jun 22, 2020 at 01:30:31AM +0530, Neeraj Upadhyay wrote: >>>> Hi Paul, >>>> >>>> On 6/22/2020 1:20 AM, Paul E. McKenney wrote: >>>>> On Mon, Jun 22, 2020 at 12:07:27AM +0530, Neeraj Upadhyay wrote: >>>>>> On callback overload, we want to force quiescent state immediately, >>>>>> for the first and second fqs. Enforce the same, by including >>>>>> RCU_GP_FLAG_OVLD flag, in fqsstart check. >>>>>> >>>>>> Signed-off-by: Neeraj Upadhyay >>>>> >>>>> Good catch! >>>>> >>>>> But what did you do to verify that this change does the right thing? >>>>> >>>>> Thanx, Paul >>>>> >>>> >>>> I haven't done a runtime verification of this code path; I posted this, >>>> based on review of this code. >>> >>> My concern is that under overload, the FQS scans would happen continuously >>> rather than accelerating only the first such scan in a given grace period. >>> This would of course result in a CPU-bound grace-period kthread, which >>> users might not be all that happy with. >>> >>> Or am I missing something subtle that prevents this? >> >> Looks like under overload, only the first and second scans are accelerated? >> >> gf = 0; >> if (first_gp_fqs) { >> first_gp_fqs = false; >> gf = rcu_state.cbovld ? RCU_GP_FLAG_OVLD : 0; >> } > > Very good, it does sound like you understand this, and it matches my > analysis and passes light testing, so I queued this one. I did improve > the commit log, please check below. The added detail is helpful to people > (including ourselves, by the way) who might need to look at this commit > some time in the future. > Thanks; patch looks good; I will try to put more efforts on commit log for future patches. > If you have an x86 system lying around, running rcutorture is quite > straightforward. Non-x86 systems can also run rcutorture, if nothing > else by using modprobe and rmmod as described here: > > https://paulmck.livejournal.com/57769.html > > The scripting described in the latter part of this document has worked > on ARMv8 and PowerPC, and might still work for all I know. I will set it up at my end; the livejournal is pretty detailed! Thanks for sharing this! Thanks Neeraj > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 9482524d7dd0aea5d32a6efa2979223eea07c029 > Author: Neeraj Upadhyay > Date: Mon Jun 22 00:07:27 2020 +0530 > > rcu/tree: Force quiescent state on callback overload > > On callback overload, it is necessary to quickly detect idle CPUs, > and rcu_gp_fqs_check_wake() checks for this condition. Unfortunately, > the code following the call to this function does not repeat this check, > which means that in reality no actual quiescent-state forcing, instead > only a couple of quick and pointless wakeups at the beginning of the > grace period. > > This commit therefore adds a check for the RCU_GP_FLAG_OVLD flag in > the post-wakeup "if" statement in rcu_gp_fqs_loop(). > > Signed-off-by: Neeraj Upadhyay > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index d0988a1..6226bfb 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1865,7 +1865,7 @@ static void rcu_gp_fqs_loop(void) > break; > /* If time for quiescent-state forcing, do it. */ > if (!time_after(rcu_state.jiffies_force_qs, jiffies) || > - (gf & RCU_GP_FLAG_FQS)) { > + (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) { > trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, > TPS("fqsstart")); > rcu_gp_fqs(first_gp_fqs); > -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation