From patchwork Tue Nov 22 01:04:20 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: 13051743 X-Patchwork-Delegate: kuba@kernel.org 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 30EA9C47089 for ; Tue, 22 Nov 2022 01:04:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232027AbiKVBEa (ORCPT ); Mon, 21 Nov 2022 20:04:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44860 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232004AbiKVBE1 (ORCPT ); Mon, 21 Nov 2022 20:04:27 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7E04911A12; Mon, 21 Nov 2022 17:04:26 -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 ams.source.kernel.org (Postfix) with ESMTPS id 2B273B818DF; Tue, 22 Nov 2022 01:04:25 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3BA9BC4315E; 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=82QgUkdn96zAlaY+UFpNNtcTMhQgVxk6sdFaelyo3zA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=oj2o0MV7bWppKYphpTZpnE345/Ekq1qeCL51DEXRnLTr9ODMUfEY6Po0mW1vDwE0N ZKt1Nr856cCvvre9rHNL7oio8FbnDnddN3xQDc/PpWXjmQUzB9Bbo4807N7jvKa+AX 87o4Tj/mVIAeC27B9POj/7hSsfsa5wE9QYk4sIycuuSeBgVhHY6Yc/Graw+BLBgdYe o8jkA8DFvGrnVI0c29rXdES4mjK6TiEOu0d3mqMORsQQgZ4cMEjqKNZT8H6/+6MXxU x4aRyTkMoQsvQgE7SOCx0yMQZ8zK5oriDMDd+lM5jCpeuQ8ZGIzm6AtDJsyBNP8HFk QSv+x/5jDzJLw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 8CE7F5C18FF; 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 Ahern , "David S. Miller" , Eric Dumazet , Hideaki YOSHIFUJI , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, "Paul E . McKenney" Subject: [PATCH v2 rcu 15/16] net: Use call_rcu_flush() for dst_release() Date: Mon, 21 Nov 2022 17:04:20 -0800 Message-Id: <20221122010421.3799681-15-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: netdev@vger.kernel.org X-Patchwork-Delegate: kuba@kernel.org From: "Joel Fernandes (Google)" In a networking test on ChromeOS, kernels built with the new CONFIG_RCU_LAZY=y Kconfig option fail a networking test in the teardown phase. This failure may be reproduced as follows: ip netns del The CONFIG_RCU_LAZY=y Kconfig option was introduced by earlier commits in this series for the benefit of certain battery-powered systems. 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. Returning to the test failure, use of ftrace showed that this failure cause caused by the aadded delays due to this new lazy behavior of call_rcu() in kernels built with CONFIG_RCU_LAZY=y. Therefore, make dst_release() use call_rcu_flush() in order to revert to the old test-failure-free behavior. Signed-off-by: Joel Fernandes (Google) Cc: David Ahern Cc: "David S. Miller" Cc: Eric Dumazet Cc: Hideaki YOSHIFUJI Cc: Jakub Kicinski Cc: Paolo Abeni Cc: Signed-off-by: Paul E. McKenney --- net/core/dst.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/core/dst.c b/net/core/dst.c index bc9c9be4e0801..15b16322703f4 100644 --- a/net/core/dst.c +++ b/net/core/dst.c @@ -174,7 +174,7 @@ void dst_release(struct dst_entry *dst) net_warn_ratelimited("%s: dst:%p refcnt:%d\n", __func__, dst, newrefcnt); if (!newrefcnt) - call_rcu(&dst->rcu_head, dst_destroy_rcu); + call_rcu_flush(&dst->rcu_head, dst_destroy_rcu); } } EXPORT_SYMBOL(dst_release);