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=-7.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 05869C433E0 for ; Fri, 22 Jan 2021 15:31:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BCE3D23A84 for ; Fri, 22 Jan 2021 15:31:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728960AbhAVPbH (ORCPT ); Fri, 22 Jan 2021 10:31:07 -0500 Received: from mail.kernel.org ([198.145.29.99]:51806 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729109AbhAVP3g (ORCPT ); Fri, 22 Jan 2021 10:29:36 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 86B1E23AA3; Fri, 22 Jan 2021 15:28:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1611329335; bh=9ldXAsBajJcOkkUbYctJnr9HIysDXrYHwaQqogk30ac=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=W88fzHVQM+jkXCokbspmzzReL5QnMNWA1enC9NXfyscIRNA7z5mJJ6O6OgMUuyytg AvcWEAKD0r29DEhTtGdUBf7aBOkCpRcswhPxlxW1CL0g7ubRjRlDOwX/AEkoeJ1Lnf t0+NF9RNN4o1TK1Rf0hjR94r7TuZBHgoKlCWRGeCoB7wXLXhcyn1UXn+DAd1xYliGb DXtVyWYE6NNf0Lf3GT7/dFo77z/HEciPMWRZQLgXYOjW0Ycg4GImcRd0oggRmu9ULj qS7kkz3GmiQc1B58bKoQUUINRLfH7lZGDK/EizDVOL5gfEao7tkq4pQisMvHeRVAzt kPkN2ZlCunnSw== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 5AB953522649; Fri, 22 Jan 2021 07:28:55 -0800 (PST) Date: Fri, 22 Jan 2021 07:28:55 -0800 From: "Paul E. McKenney" To: Sebastian Andrzej Siewior Cc: "Uladzislau Rezki (Sony)" , LKML , RCU , Michael Ellerman , Andrew Morton , Daniel Axtens , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Peter Zijlstra , Michal Hocko , Thomas Gleixner , "Theodore Y . Ts'o" , Oleksiy Avramchenko Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument Message-ID: <20210122152855.GE2743@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20210120162148.1973-1-urezki@gmail.com> <20210120195757.3lgjrpvmzjvb2nce@linutronix.de> <20210120215403.GH2743@paulmck-ThinkPad-P72> <20210122111733.tcwfl43akypz3x42@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210122111733.tcwfl43akypz3x42@linutronix.de> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 22, 2021 at 12:17:33PM +0100, Sebastian Andrzej Siewior wrote: > On 2021-01-20 13:54:03 [-0800], Paul E. McKenney wrote: > > > > +// Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock() > > > > +// state specified by flags. If can_alloc is true, the caller must > > > > +// be schedulable and not be holding any locks or mutexes that might be > > > > +// acquired by the memory allocator or anything that it might invoke. > > > > +// Returns true if ptr was successfully recorded, else the caller must > > > > +// use a fallback. > > > > > > The whole RCU department is getting swamped by the // comments. Can't we > > > have proper kernel doc and /* */ style comments like the remaining part > > > of the kernel? > > > > Because // comments are easier to type and take up less horizontal space. > > As for the typing I could try to sell you > ab // /* > > for your .vimrc and then // would become /* ;) As for the > horizontal space, I don't have currently anything in my shop. I'm sorry. ;-) > > Also, this kvfree_call_rcu_add_ptr_to_bulk() function is local to > > kvfree_rcu(), and we don't normally docbook-ify such functions. > > I didn't mean to promote using docbook to use every. For instance if you > look at kernel/trace/trace.c, there are no // comments around, just /* > style, even for things like tracing_selftest_running. > > Basically I was curious if I could learn where this // is coming and if > I could stop it. Because they are now allowed and because they make my life easier as noted above. Also in-function comment blocks are either one line or two lines shorter. Yeah, they look strange at first, but it is not that hard to get used to them. After all, I did manage to get used to the /* */ comment style shortly after first encountering it. ;-) > > > > static inline bool > > > > -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr) > > > > +add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp, > > > > + unsigned long *flags, void *ptr, bool can_alloc) > > > > { > > > > struct kvfree_rcu_bulk_data *bnode; > > > > int idx; > > > > > > > > - if (unlikely(!krcp->initialized)) > > > > + *krcp = krc_this_cpu_lock(flags); > > > > + if (unlikely(!(*krcp)->initialized)) > > > > return false; > > > > > > > > - lockdep_assert_held(&krcp->lock); > > > > idx = !!is_vmalloc_addr(ptr); > > > > > > > > /* Check if a new block is required. */ > > > > - if (!krcp->bkvhead[idx] || > > > > - krcp->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) { > > > > - bnode = get_cached_bnode(krcp); > > > > - /* Switch to emergency path. */ > > > > + if (!(*krcp)->bkvhead[idx] || > > > > + (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) { > > > > + bnode = get_cached_bnode(*krcp); > > > > + if (!bnode && can_alloc) { > > > > + krc_this_cpu_unlock(*krcp, *flags); > > > > + bnode = (struct kvfree_rcu_bulk_data *) > > > > > > There is no need for this cast. > > > > Without it, gcc version 7.5.0 says: > > > > warning: assignment makes pointer from integer without a cast > > > > I'm sorry. I forgot the part where __get_free_page() does not return > (void *). > But maybe it should given that free_pages() casts that long back to > (void *) and __get_free_pages() -> page_address() returns (void *) > which is then casted long. No argument here. Then again, I am not the one in need of convincing. There are use cases like this from pte_alloc_one_kernel(): unsigned long page = __get_free_page(GFP_DMA); But a quick look indicates that they are in the minority. > > > > + __get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN); > > > > + *krcp = krc_this_cpu_lock(flags); > > > > > > so if bnode is NULL you could retry get_cached_bnode() since it might > > > have been filled (given preemption or CPU migration changed something). > > > Judging from patch #3 you think that a CPU migration is a bad thing. But > > > why? > > > > So that the later "(*krcp)->bkvhead[idx] = bnode" assignment associates > > it with the correct CPU. > > > > Though now that you mention it, couldn't the following happen? > > > > o Task A on CPU 0 notices that allocation is needed, so it > > drops the lock disables migration, and sleeps while > > allocating. > > > > o Task B on CPU 0 does the same. > > > > o The two tasks wake up in some order, and the second one > > causes trouble at the "(*krcp)->bkvhead[idx] = bnode" > > assignment. > > Yes it could, good point. > I would really recommend using migrate_disable() at a minimum and only > if it is really needed. It is more expensive than preempt_disable() and > it isn't exactly good in terms of scheduling since the task is run able > but restricted to a specific CPU. > If it is unavoidable it is unavoidable but in this case I wouldn't use > migrate_disable() but re-evaluate the situation after the allocation. I could imagine the following alternatives: o Acquire the old CPU's lock despite having been migrated. If the above race happened, put the extra page in the per-CPU cache. As Uladzislau notes, this would require some sort of periodic cleanup that would be good to avoid. o As now, which can result in an unfilled page, though only in an uncommon race condition. (Uladzislau convinced me that this was a good approach some months ago, and the fact that he cannot make it happen easily does add some weight to his argument.) o Use migrate_disable(). Other ideas? Thanx, Paul > > Uladzislau, do we need to recheck "!(*krcp)->bkvhead[idx]" just after > > the migrate_enable()? Along with the KVFREE_BULK_MAX_ENTR check? > > > > Thanx, Paul > > Sebastian