From patchwork Tue Nov 22 01:04:19 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: 13051753 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 1DE69C4321E for ; Tue, 22 Nov 2022 01:04:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232029AbiKVBEb (ORCPT ); Mon, 21 Nov 2022 20:04:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44806 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231956AbiKVBE0 (ORCPT ); Mon, 21 Nov 2022 20:04:26 -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 1C42C1C901; Mon, 21 Nov 2022 17:04:25 -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 75F0061511; Tue, 22 Nov 2022 01:04:24 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 33D5FC43156; 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=QNpGjRTwvCwUOFj/pR6pze23etHEHGqQMnnOZBmIHF0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fjaKQJ3eSDwRn22hicp3w41Zl2GWI3YjHGmDWDJVyAPiyxrZOPD78A4UgUF/RxlKB mXGO2rgS6XfFXotIqbsRqz4o+pd1q/Hs692e4No5+HTCd0nOByQWtv9EHTTE1hc24X CFJQp86yxs1lzaKlkx+DuntsELItJ0tfxyeN6coVE+eJQ+MD+thR7MIPPb4Ie090Xp UZm6mR87a4Top0HJCaCJNNz08fh8MG6AgBOxWL2doq4PriTEJZZbwiURgI4Xy6Flqh qn4dfosyXjLIxsH4W3GBjX+5zNJCur7P2JuBr9Pit8CDSkARL0mDq5+BiumQOmTzzD nUWm6MnXK4CZA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 8B2805C1695; 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, "Joel Fernandes (Google)" , David Howells , Marc Dionne , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , linux-afs@lists.infradead.org, netdev@vger.kernel.org, "Paul E . McKenney" Subject: [PATCH v2 rcu 14/16] rxrpc: Use call_rcu_flush() instead of call_rcu() Date: Mon, 21 Nov 2022 17:04:19 -0800 Message-Id: <20221122010421.3799681-14-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: "Joel Fernandes (Google)" 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 rxrpc_kill_connection(), which sometimes does a wakeup that should not be unduly delayed. Therefore, make rxrpc_kill_connection() use call_rcu_flush() in order to revert to the old behavior. Signed-off-by: Joel Fernandes (Google) Cc: David Howells Cc: Marc Dionne Cc: "David S. Miller" Cc: Eric Dumazet Cc: Jakub Kicinski Cc: Paolo Abeni Cc: Cc: Signed-off-by: Paul E. McKenney --- net/rxrpc/conn_object.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/rxrpc/conn_object.c b/net/rxrpc/conn_object.c index 22089e37e97f0..fdcfb509cc443 100644 --- a/net/rxrpc/conn_object.c +++ b/net/rxrpc/conn_object.c @@ -253,7 +253,7 @@ void rxrpc_kill_connection(struct rxrpc_connection *conn) * must carry a ref on the connection to prevent us getting here whilst * it is queued or running. */ - call_rcu(&conn->rcu, rxrpc_destroy_connection); + call_rcu_flush(&conn->rcu, rxrpc_destroy_connection); } /*