From patchwork Tue Nov 22 01:04:18 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. McKenney" X-Patchwork-Id: 13051752 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 196AEC47088 for ; Tue, 22 Nov 2022 01:04:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232020AbiKVBE3 (ORCPT ); Mon, 21 Nov 2022 20:04:29 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44780 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231946AbiKVBEZ (ORCPT ); Mon, 21 Nov 2022 20:04:25 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D3F7C11A12; Mon, 21 Nov 2022 17:04:24 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 13F7F6151F; Tue, 22 Nov 2022 01:04:24 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3228FC43154; Tue, 22 Nov 2022 01:04:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1669079063; bh=JDkX7Fc0DOy+yBhbkuqH9S9Ejfgbk7JA8huLd1R6z8Q=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ERo9qt7uO7GVBBnrX8o2yXKM/JykcLBdKAbt/RQK4nUKXF1KXqh6rH97v9r5k9ZXA WumeDeAgQ6u2DwTKhLbgEUQi9W/b1FOGQkAEq75C51k0vVc6/BAUAIEWz/HBMK4vI5 l0puY9/6h3ocIVuzk2IqMT5KJCe+V61HRQok8UROZEnccnPyNxpaauTlmY/L4CEmXY 9hfAa7Osg5YjfWoq7Y0ORylTq6OuP+ZtK1SsgJyIGpu/HQWPMGq/ce3G253xKj4Ctz NVDwNSK7+OAzWB7T5QrsS8ecsRwC2aSLbLC1ByBPcM7Uiq7FBYS8zMeUuF27bmV1bV iYx8FNDVQU4bw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 8954E5C155D; Mon, 21 Nov 2022 17:04:22 -0800 (PST) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, Uladzislau Rezki , Joel Fernandes , Tejun Heo , Lai Jiangshan , "Paul E . McKenney" Subject: [PATCH v2 rcu 13/16] workqueue: Make queue_rcu_work() use call_rcu_flush() Date: Mon, 21 Nov 2022 17:04:18 -0800 Message-Id: <20221122010421.3799681-13-paulmck@kernel.org> X-Mailer: git-send-email 2.31.1.189.g2e36527f23 In-Reply-To: <20221122010408.GA3799268@paulmck-ThinkPad-P17-Gen-1> References: <20221122010408.GA3799268@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org From: Uladzislau Rezki Earlier commits in this series allow battery-powered systems to build their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option. This Kconfig option causes call_rcu() to delay its callbacks in order to batch them. This means that a given RCU grace period covers more callbacks, thus reducing the number of grace periods, in turn reducing the amount of energy consumed, which increases battery lifetime which can be a very good thing. This is not a subtle effect: In some important use cases, the battery lifetime is increased by more than 10%. This CONFIG_RCU_LAZY=y option is available only for CPUs that offload callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y. Delaying callbacks is normally not a problem because most callbacks do nothing but free memory. If the system is short on memory, a shrinker will kick all currently queued lazy callbacks out of their laziness, thus freeing their memory in short order. Similarly, the rcu_barrier() function, which blocks until all currently queued callbacks are invoked, will also kick lazy callbacks, thus enabling rcu_barrier() to complete in a timely manner. However, there are some cases where laziness is not a good option. For example, synchronize_rcu() invokes call_rcu(), and blocks until the newly queued callback is invoked. It would not be a good for synchronize_rcu() to block for ten seconds, even on an idle system. Therefore, synchronize_rcu() invokes call_rcu_flush() instead of call_rcu(). The arrival of a non-lazy call_rcu_flush() callback on a given CPU kicks any lazy callbacks that might be already queued on that CPU. After all, if there is going to be a grace period, all callbacks might as well get full benefit from it. Yes, this could be done the other way around by creating a call_rcu_lazy(), but earlier experience with this approach and feedback at the 2022 Linux Plumbers Conference shifted the approach to call_rcu() being lazy with call_rcu_flush() for the few places where laziness is inappropriate. And another call_rcu() instance that cannot be lazy is the one in queue_rcu_work(), given that callers to queue_rcu_work() are not necessarily OK with long delays. Therefore, make queue_rcu_work() use call_rcu_flush() in order to revert to the old behavior. Signed-off-by: Uladzislau Rezki Signed-off-by: Joel Fernandes (Google) Cc: Tejun Heo Cc: Lai Jiangshan Signed-off-by: Paul E. McKenney --- kernel/workqueue.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 7cd5f5e7e0a1b..b4b0e828b529e 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -1771,7 +1771,7 @@ bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork) if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) { rwork->wq = wq; - call_rcu(&rwork->rcu, rcu_work_rcufn); + call_rcu_flush(&rwork->rcu, rcu_work_rcufn); return true; }