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=-19.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 A93FAC433E0 for ; Wed, 6 Jan 2021 01:27:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7D02422D04 for ; Wed, 6 Jan 2021 01:27:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726215AbhAFB1B (ORCPT ); Tue, 5 Jan 2021 20:27:01 -0500 Received: from mail.kernel.org ([198.145.29.99]:33588 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725730AbhAFB07 (ORCPT ); Tue, 5 Jan 2021 20:26:59 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id BD30A22D01; Wed, 6 Jan 2021 01:26:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1609896378; bh=jN9NdEnuR9c2ErjPZ57T2P7JYMhtHrOC4839Om1h31g=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=UTbkiyOtJdppMZntpKFR0Ob0wiPpRt3IIGW7kvYt5MLfpOJEnEJG9pMaqwuVZ83e+ PGA1bPh2t9XFot+tDrlMCfTLvKV2S3X7TlkRJ2HKnY1tpsBYYYxuHS6XUa5IVnxocY EKMFYbf1vhszsvd1YZFerbOQsI+lUHpzIbIuug46mGVxiQW9ofwN2CtPiUHRAfnrJZ dCwZ7LpEpnD2PdBqahl7xlu1BtGr2V7nE3GHIUpWTpjJrEGT2SDC5MSMZ8D5K2g2x1 rYjGKdWMeRg+OcZlcLqqHeVsj6YwnTx+hwiBwVnbxkuq4AjqmYdwhR/rBZBTIV8uA2 aOAz84UJiJ1lg== From: paulmck@kernel.org To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "Paul E . McKenney" Subject: [PATCH tip/core/rcu 1/6] rcu/tree: Make rcu_do_batch count how many callbacks were executed Date: Tue, 5 Jan 2021 17:26:12 -0800 Message-Id: <20210106012617.14122-1-paulmck@kernel.org> X-Mailer: git-send-email 2.9.5 In-Reply-To: <20210106012541.GA13972@paulmck-ThinkPad-P72> References: <20210106012541.GA13972@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Joel Fernandes (Google)" The rcu_do_batch() function extracts the ready-to-invoke callbacks from the rcu_segcblist located in the ->cblist field of the current CPU's rcu_data structure. These callbacks are first moved to a local (unsegmented) rcu_cblist. The rcu_do_batch() function then uses this rcu_cblist's ->len field to count how many CBs it has invoked, but it does so by counting that field down from zero. Finally, this function negates the value in this ->len field (resulting in a positive number) and subtracts the result from the ->len field of the current CPU's ->cblist field. Except that it is sometimes necessary for rcu_do_batch() to stop invoking callbacks mid-stream, despite there being more ready to invoke, for example, if a high-priority task wakes up. In this case the remaining not-yet-invoked callbacks are requeued back onto the CPU's ->cblist, but remain in the ready-to-invoke segment of that list. As above, the negative of the local rcu_cblist's ->len field is still subtracted from the ->len field of the current CPU's ->cblist field. The design of counting down from 0 is confusing and error-prone, plus use of a positive count will make it easier to provide a uniform and consistent API to deal with the per-segment counts that are added later in this series. For example, rcu_segcblist_extract_done_cbs() can unconditionally populate the resulting unsegmented list's ->len field during extraction. This commit therefore explicitly counts how many callbacks were executed in rcu_do_batch() itself, counting up from zero, and then uses that to update the per-CPU segcb list's ->len field, without relying on the downcounting of rcl->len from zero. Reviewed-by: Frederic Weisbecker Reviewed-by: Neeraj Upadhyay Signed-off-by: Joel Fernandes (Google) Signed-off-by: Paul E. McKenney --- kernel/rcu/rcu_segcblist.c | 2 +- kernel/rcu/rcu_segcblist.h | 1 + kernel/rcu/tree.c | 11 +++++------ 3 files changed, 7 insertions(+), 7 deletions(-) diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c index 2d2a6b6b9..bb246d8 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 492262b..1d2d614 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 40e5e3d..cc6f379 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2434,7 +2434,7 @@ static void rcu_do_batch(struct rcu_data *rdp) const bool offloaded = 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. */ @@ -2479,6 +2479,7 @@ 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); @@ -2492,15 +2493,14 @@ 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; if (unlikely(tlimit)) { /* only call local_clock() every 32 callbacks */ - if (likely((-rcl.len & 31) || local_clock() < tlimit)) + if (likely((count & 31) || local_clock() < tlimit)) continue; /* Exceeded the time limit, so leave. */ break; @@ -2517,7 +2517,6 @@ 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()); @@ -2525,7 +2524,7 @@ 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); -- 2.9.5