From patchwork Tue Nov 22 01:04:13 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. 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McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, "Joel Fernandes (Google)" , "Paul E . McKenney" , Dennis Zhou , Tejun Heo , Christoph Lameter , linux-mm@kvack.org Subject: [PATCH v2 rcu 08/16] percpu-refcount: Use call_rcu_flush() for atomic switch Date: Mon, 21 Nov 2022 17:04:13 -0800 Message-Id: <20221122010421.3799681-8-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 ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1669079066; a=rsa-sha256; cv=none; b=qzusrM8Aa4M8qnW96hmce8I/XVIshkVjVmDVt0j/LFTw7E6RSGZT8vTcDf0BQlyIl8+Ekk weSu8bxrPygC4wWNllgZin+2HJHiNwd8xsynRkhnq0wWPQBgKIb0kFiMJUZcHK4qs0fSa8 38ugno8MtgRB4uEOPbgYUxHRxQe00SU= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=ouiCNi1M; spf=pass (imf30.hostedemail.com: domain of "SRS0=hi7Z=3W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 139.178.84.217 as permitted sender) smtp.mailfrom="SRS0=hi7Z=3W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org"; dmarc=pass (policy=none) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1669079066; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=T+B3Wd3V/zk3IHc0EsSAng9ddSG+4DT5R4CU+3RmirU=; b=Ry8cq8XTKQqW7sdhWxdm1B155eFtO4L++fUp5buEYmcgmCKb6PJ7OQGiwBaG2fbzRsRGiZ TRjpswTALhsIViOHVsDo7V1ICjzGuQEtCjXzqJrXmmz8q69UytJSs77poKp/ZcifyEjAU4 cOt8vbJ/Mq//QfLeK4wlnTdNuhW49uY= X-Rspam-User: X-Rspamd-Queue-Id: DA2B280011 Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=ouiCNi1M; spf=pass (imf30.hostedemail.com: domain of "SRS0=hi7Z=3W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 139.178.84.217 as permitted sender) smtp.mailfrom="SRS0=hi7Z=3W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org"; dmarc=pass (policy=none) header.from=kernel.org X-Stat-Signature: 68ytpdisaa88aywg4og5xdkx4hoomyd9 X-Rspamd-Server: rspam10 X-HE-Tag: 1669079065-275030 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 callbacks. 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 on the percpu refcounter's "per-CPU to atomic switch" code path, which uses RCU when switching to atomic mode. The enqueued callback wakes up waiters waiting in the percpu_ref_switch_waitq. Allowing this callback to be lazy would result in unacceptable slowdowns for users of per-CPU refcounts, such as blk_pre_runtime_suspend(). Therefore, make __percpu_ref_switch_to_atomic() use call_rcu_flush() in order to revert to the old behavior. Signed-off-by: Joel Fernandes (Google) Signed-off-by: Paul E. McKenney Cc: Dennis Zhou Cc: Tejun Heo Cc: Christoph Lameter Cc: --- lib/percpu-refcount.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index e5c5315da2741..65c58a029297d 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -230,7 +230,8 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref, percpu_ref_noop_confirm_switch; percpu_ref_get(ref); /* put after confirmation */ - call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu); + call_rcu_flush(&ref->data->rcu, + percpu_ref_switch_to_atomic_rcu); } static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)