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=-11.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, NICE_REPLY_A,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable 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 35D01C43457 for ; Wed, 14 Oct 2020 15:06:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B55FB21582 for ; Wed, 14 Oct 2020 15:06:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=mg.codeaurora.org header.i=@mg.codeaurora.org header.b="H9kRTgb5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731578AbgJNPGp (ORCPT ); Wed, 14 Oct 2020 11:06:45 -0400 Received: from z5.mailgun.us ([104.130.96.5]:56759 "EHLO z5.mailgun.us" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726334AbgJNPGp (ORCPT ); Wed, 14 Oct 2020 11:06:45 -0400 DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=mg.codeaurora.org; q=dns/txt; s=smtp; t=1602688003; h=Content-Transfer-Encoding: Content-Type: In-Reply-To: MIME-Version: Date: Message-ID: From: References: Cc: To: Subject: Sender; bh=e2kGAgVHzh9Hqu3iJnnfFGTiII18swYy+tQKbtPXPm8=; b=H9kRTgb5TV3cwDEkyB+xkpnPGiCfhb+XSQUkocVCWeUKc1C2sFXCFf1VDu08tB0hgqLiEpXJ c5VLj577CTVwDB9nLPJhVvO+Q3SKxTqGdQV5i7FV34A8+0jG6hiSQLHBmz/3/FU8VkPvkX6X rp3JUDgGH2rS3ZPvU/x2qNuQ+Oo= X-Mailgun-Sending-Ip: 104.130.96.5 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-n05.prod.us-west-2.postgun.com with SMTP id 5f8713f083370fa1c142a4ad (version=TLS1.2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256); Wed, 14 Oct 2020 15:06:24 GMT Sender: neeraju=codeaurora.org@mg.codeaurora.org Received: by smtp.codeaurora.org (Postfix, from userid 1001) id 0EAB3C433FE; Wed, 14 Oct 2020 15:06:24 +0000 (UTC) Received: from [192.168.0.102] (unknown [49.206.34.248]) (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 8B670C433F1; Wed, 14 Oct 2020 15:06:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 8B670C433F1 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=fail smtp.mailfrom=neeraju@codeaurora.org Subject: Re: [PATCH v6 1/4] rcu/tree: Make rcu_do_batch count how many callbacks were executed To: "Joel Fernandes (Google)" , linux-kernel@vger.kernel.org Cc: Ingo Molnar , Josh Triplett , Lai Jiangshan , Madhuparna Bhowmik , Mathieu Desnoyers , neeraj.iitr10@gmail.com, "Paul E. McKenney" , rcu@vger.kernel.org, Steven Rostedt , "Uladzislau Rezki (Sony)" References: <20200923152211.2403352-1-joel@joelfernandes.org> <20200923152211.2403352-2-joel@joelfernandes.org> From: Neeraj Upadhyay Message-ID: Date: Wed, 14 Oct 2020 20:36:16 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.1 MIME-Version: 1.0 In-Reply-To: <20200923152211.2403352-2-joel@joelfernandes.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/23/2020 8:52 PM, Joel Fernandes (Google) wrote: > Currently, rcu_do_batch() depends on the unsegmented callback list's len field > to know how many CBs are executed. This fields counts down from 0 as CBs are > dequeued. It is possible that all CBs could not be run because of reaching > limits in which case the remaining unexecuted callbacks are requeued in the > CPU's segcblist. > > The number of callbacks that were not requeued are then the negative count (how > many CBs were run) stored in the rcl->len which has been counting down on every > dequeue. This negative count is then added to the per-cpu segmented callback > list's to correct its count. > > Such a design works against future efforts to track the length of each segment > of the segmented callback list. The reason is because > rcu_segcblist_extract_done_cbs() will be populating the unsegmented callback > list's length field (rcl->len) during extraction. > > Also, the design of counting down from 0 is confusing and error-prone IMHO. > > This commit therefore explicitly counts have many callbacks were executed in > rcu_do_batch() itself, and uses that to update the per-CPU segcb list's ->len > field, without relying on the negativity of rcl->len. > > Signed-off-by: Joel Fernandes (Google) > --- > kernel/rcu/rcu_segcblist.c | 2 +- > kernel/rcu/rcu_segcblist.h | 1 + > kernel/rcu/tree.c | 9 ++++----- > 3 files changed, 6 insertions(+), 6 deletions(-) > > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c > index 2d2a6b6b9dfb..bb246d8c6ef1 100644 > --- a/kernel/rcu/rcu_segcblist.c > +++ b/kernel/rcu/rcu_segcblist.c > @@ -95,7 +95,7 @@ static void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v) > * This increase is fully ordered with respect to the callers accesses > * both before and after. > */ > -static void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v) > +void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v) > { > #ifdef CONFIG_RCU_NOCB_CPU > smp_mb__before_atomic(); /* Up to the caller! */ > diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h > index 5c293afc07b8..b90725f81d77 100644 > --- a/kernel/rcu/rcu_segcblist.h > +++ b/kernel/rcu/rcu_segcblist.h > @@ -76,6 +76,7 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg) > } > > void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp); > +void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v); > void rcu_segcblist_init(struct rcu_segcblist *rsclp); > void rcu_segcblist_disable(struct rcu_segcblist *rsclp); > void rcu_segcblist_offload(struct rcu_segcblist *rsclp); > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 7623128d0020..50af465729f4 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2427,7 +2427,7 @@ static void rcu_do_batch(struct rcu_data *rdp) > rcu_segcblist_is_offloaded(&rdp->cblist); > struct rcu_head *rhp; > struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl); > - long bl, count; > + long bl, count = 0; > long pending, tlimit = 0; > > /* If no callbacks are ready, just return. */ > @@ -2472,6 +2472,7 @@ static void rcu_do_batch(struct rcu_data *rdp) > for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) { > rcu_callback_t f; > > + count++; > debug_rcu_head_unqueue(rhp); > > rcu_lock_acquire(&rcu_callback_map); > @@ -2485,9 +2486,8 @@ static void rcu_do_batch(struct rcu_data *rdp) > > /* > * Stop only if limit reached and CPU has something to do. > - * Note: The rcl structure counts down from zero. > */ > - if (-rcl.len >= bl && !offloaded && > + if (count >= bl && !offloaded && > (need_resched() || > (!is_idle_task(current) && !rcu_is_callbacks_kthread()))) > break; Update below usage of -rcl.len also? if (likely((-rcl.len & 31) || local_clock() < tlimit)) Thanks Neeraj > @@ -2510,7 +2510,6 @@ static void rcu_do_batch(struct rcu_data *rdp) > > local_irq_save(flags); > rcu_nocb_lock(rdp); > - count = -rcl.len; > rdp->n_cbs_invoked += count; > trace_rcu_batch_end(rcu_state.name, count, !!rcl.head, need_resched(), > is_idle_task(current), rcu_is_callbacks_kthread()); > @@ -2518,7 +2517,7 @@ static void rcu_do_batch(struct rcu_data *rdp) > /* Update counts and requeue any remaining callbacks. */ > rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl); > smp_mb(); /* List handling before counting for rcu_barrier(). */ > - rcu_segcblist_insert_count(&rdp->cblist, &rcl); > + rcu_segcblist_add_len(&rdp->cblist, -count); > > /* Reinstate batch limit if we have worked down the excess. */ > count = rcu_segcblist_n_cbs(&rdp->cblist); > -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation