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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 98D27C46460 for ; Tue, 14 Aug 2018 16:49:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0B6792172B for ; Tue, 14 Aug 2018 16:49:51 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0B6792172B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=goodmis.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732767AbeHNThn (ORCPT ); Tue, 14 Aug 2018 15:37:43 -0400 Received: from mail.kernel.org ([198.145.29.99]:37440 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732280AbeHNThn (ORCPT ); Tue, 14 Aug 2018 15:37:43 -0400 Received: from gandalf.local.home (cpe-66-24-56-78.stny.res.rr.com [66.24.56.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B899C21728; Tue, 14 Aug 2018 16:49:46 +0000 (UTC) Date: Tue, 14 Aug 2018 12:49:45 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: joel@joelfernandes.org, mathieu.desnoyers@efficios.com, peterz@infradead.org, tj@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC] Make call_srcu() available during very early boot Message-ID: <20180814124945.5cf1753f@gandalf.local.home> In-Reply-To: <20180814162448.GA20414@linux.vnet.ibm.com> References: <20180814162448.GA20414@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 14 Aug 2018 09:24:48 -0700 "Paul E. McKenney" wrote: > Event tracing is moving to SRCU in order to take advantage of the fact > that SRCU may be safely used from idle and even offline CPUs. However, > event tracing can invoke call_srcu() very early in the boot process, > even before workqueue_init_early() is invoked (let alone rcu_init()). > Therefore, call_srcu()'s attempts to queue work fail miserably. > > This commit therefore detects this situation, and refrains from attempting > to queue work before rcu_init() time, but does everything else that it > would have done, and in addition, adds the srcu_struct to a global list. > The rcu_init() function now invokes a new srcu_init() function, which > is empty if CONFIG_SRCU=n. Otherwise, srcu_init() queues work for > each srcu_struct on the list. This all happens early enough in boot > that there is but a single CPU with interrupts disabled, which allows > synchronization to be dispensed with. > > Of course, the queued work won't actually be invoked until after > workqueue_init() is invoked, which happens shortly after the scheduler > is up and running. This means that although call_srcu() may be invoked > any time after per-CPU variables have been set up, there is still a very > narrow window when synchronize_srcu() won't work, and this window > extends from the time that the scheduler starts until the time that > workqueue_init() returns. This can be fixed in a manner similar to > the fix for synchronize_rcu_expedited() and friends, but until someone > actually needs to use synchronize_srcu() during this window, this fix > is added churn for no benefit. > > Finally, note that Tree SRCU's new srcu_init() function invokes > queue_work() rather than the queue_delayed_work() function that is invoked > post-boot. The reason is that queue_delayed_work() will (as you would > expect) post a timer, and timers have not yet been initialized. So use > of queue_delayed_work() avoids the complaints about use of uninitialized You mean "So use of queue_work() avoids .." ? > spinlocks that would otherwise result. Besides, delay is in any case > provide by the aforementioned fact that the queued work won't actually > be invoked until after the scheduler is up and running. > > Requested-by: Steven Rostedt > Signed-off-by: Paul E. McKenney > > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h > index f41d2fb09f87..2b5c0822e683 100644 > --- a/include/linux/srcutiny.h > +++ b/include/linux/srcutiny.h > @@ -36,6 +36,7 @@ struct srcu_struct { > struct rcu_head *srcu_cb_head; /* Pending callbacks: Head. */ > struct rcu_head **srcu_cb_tail; /* Pending callbacks: Tail. */ > struct work_struct srcu_work; /* For driving grace periods. */ > + struct list_head srcu_boot_entry; /* Early-boot callbacks. */ I really don't like increasing the size of a structure for a field that is hardly ever used. Is there a way we could make a union, or reuse one of the other fields, as we know that synchronize_srcu() can't be used yet (and if it is, either warn, or just make it a nop). And when we call srcu_init() and remove the srcu_struct from the list, we can then initialize whatever we used as the temporary boot up list field. srcu_init is called when we are still running only one CPU correct? > #ifdef CONFIG_DEBUG_LOCK_ALLOC > struct lockdep_map dep_map; > #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ > @@ -48,6 +49,7 @@ void srcu_drive_gp(struct work_struct *wp); > .srcu_wq = __SWAIT_QUEUE_HEAD_INITIALIZER(name.srcu_wq), \ > .srcu_cb_tail = &name.srcu_cb_head, \ > .srcu_work = __WORK_INITIALIZER(name.srcu_work, srcu_drive_gp), \ > + .srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry), \ > __SRCU_DEP_MAP_INIT(name) \ > } > > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h > index 745d4ca4dd50..86ad97111315 100644 > --- a/include/linux/srcutree.h > +++ b/include/linux/srcutree.h > @@ -94,6 +94,7 @@ struct srcu_struct { > /* callback for the barrier */ > /* operation. */ > struct delayed_work work; > + struct list_head srcu_boot_entry; /* Early-boot callbacks. */ > #ifdef CONFIG_DEBUG_LOCK_ALLOC > struct lockdep_map dep_map; > #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ > @@ -105,12 +106,13 @@ struct srcu_struct { > #define SRCU_STATE_SCAN2 2 > > #define __SRCU_STRUCT_INIT(name, pcpu_name) \ > - { \ > - .sda = &pcpu_name, \ > - .lock = __SPIN_LOCK_UNLOCKED(name.lock), \ > - .srcu_gp_seq_needed = 0 - 1, \ > - __SRCU_DEP_MAP_INIT(name) \ > - } > +{ \ > + .sda = &pcpu_name, \ > + .lock = __SPIN_LOCK_UNLOCKED(name.lock), \ > + .srcu_gp_seq_needed = 0 - 1, \ Interesting initialization of -1. This was there before, but still interesting none the less. > + .srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry), \ > + __SRCU_DEP_MAP_INIT(name) \ > +} > > -- Steve