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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 E5431C433E0 for ; Fri, 22 Jan 2021 11:42:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A38E823122 for ; Fri, 22 Jan 2021 11:42:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727096AbhAVLml (ORCPT ); Fri, 22 Jan 2021 06:42:41 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:53504 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728045AbhAVLSc (ORCPT ); Fri, 22 Jan 2021 06:18:32 -0500 Date: Fri, 22 Jan 2021 12:17:33 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1611314253; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=YwTRQGMy0i03jZO3dB3PKOrJQEnXm5M6Cbl9HlVv5ow=; b=yED0gM023rMSdxj6jRSDaYAczCErP0qbcKBAHB7OnuzqdueVSkP3OEVtJpMfz8GV6Bitiv DiN6oOvRfx2mAqIYseZWlsOt2tPVVEIbOKITyG9moGoFqBoZfn9/zG+7nuJMJBDPL7W1Q9 DfntvM+jqzrx/+gw9/I7rvaDjjR/86DqQ30b4Jv2keGNezI8gue3jq0rSvcbdHs0XznNUt dVzPg0ZBKX9Vv6qjIpjQuVvqLwOuJpcJ29q3/0ibCkvrpyHdoMHOfiAd+QvyW8X96xkLjJ fB/+GSpiz14duYdpsfzHncuAjdL6cYgMa+7fH6fUTzzhbNUTsbinm7dGgSHcIw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1611314253; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=YwTRQGMy0i03jZO3dB3PKOrJQEnXm5M6Cbl9HlVv5ow=; b=a7/ZJEji7l60cuGN/7n76DIsiqc37nyZDZTFCap6sq18Kw+tVHZtqkMWY/nzwUge7wQk2i y4kA3bl+yP1a/7BQ== From: Sebastian Andrzej Siewior To: "Paul E. McKenney" 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: <20210122111733.tcwfl43akypz3x42@linutronix.de> References: <20210120162148.1973-1-urezki@gmail.com> <20210120195757.3lgjrpvmzjvb2nce@linutronix.de> <20210120215403.GH2743@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210120215403.GH2743@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > > 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. > > > + __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. > 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