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=-17.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 A94A9C433DB for ; Wed, 20 Jan 2021 22:30:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 653F82220B for ; Wed, 20 Jan 2021 22:30:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726711AbhATWZx (ORCPT ); Wed, 20 Jan 2021 17:25:53 -0500 Received: from mail.kernel.org ([198.145.29.99]:60226 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732679AbhATVys (ORCPT ); Wed, 20 Jan 2021 16:54:48 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id D80BC235F9; Wed, 20 Jan 2021 21:54:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1611179643; bh=9uCIoNTqSujww0WyWHTQe652w3SNwj3GXwVl53iL7Hk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=uQ4lh3g5MNW/4HX7y+c8MMDW4iaA8LHntbZAZBtAKt84AZBdFOSRSn8/PrhGv0dHk rbubLCUw6B2xbYZ+pxprq7X81mgdky5QUH4HWcNMSyEl6MMjyzc9nPOSoI7D0fFU0l yYisLeC6WFJXuIWMHgGBOJynBhbXKTSH9ITi1lRlWVZwpz+ey8GuE9bUMmcGelctey 7DMd3ERZutupgx7ta+E9vWdrDCEPjmx84lfD8QeC+mz3ll02/a57reT93Lgo5XJ4Be sEUm0bxPWmISQx4OhX/azWFi7h48zW6axJZSZRTSkUPAQZntPgOBblZQpaZ4yvzISX LK7Qh3aekbgVQ== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id A9AD83522B82; Wed, 20 Jan 2021 13:54:03 -0800 (PST) Date: Wed, 20 Jan 2021 13:54:03 -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: <20210120215403.GH2743@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20210120162148.1973-1-urezki@gmail.com> <20210120195757.3lgjrpvmzjvb2nce@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210120195757.3lgjrpvmzjvb2nce@linutronix.de> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Wed, Jan 20, 2021 at 08:57:57PM +0100, Sebastian Andrzej Siewior wrote: > On 2021-01-20 17:21:46 [+0100], Uladzislau Rezki (Sony) wrote: > > For a single argument we can directly request a page from a caller > > context when a "carry page block" is run out of free spots. Instead > > of hitting a slow path we can request an extra page by demand and > > proceed with a fast path. > > > > A single-argument kvfree_rcu() must be invoked in sleepable contexts, > > and that its fallback is the relatively high latency synchronize_rcu(). > > Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL > > to allow limited sleeping within the memory allocator. > > > > [ paulmck: Add add_ptr_to_bulk_krc_lock header comment per Michal Hocko. ] > > Signed-off-by: Uladzislau Rezki (Sony) > > Signed-off-by: Paul E. McKenney > > --- > > kernel/rcu/tree.c | 42 ++++++++++++++++++++++++++---------------- > > 1 file changed, 26 insertions(+), 16 deletions(-) > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index e04e336bee42..2014fb22644d 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -3465,37 +3465,50 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp) > > } > > } > > > > +// 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. Also, this kvfree_call_rcu_add_ptr_to_bulk() function is local to kvfree_rcu(), and we don't normally docbook-ify such functions. > > 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 > > + __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. Uladzislau, do we need to recheck "!(*krcp)->bkvhead[idx]" just after the migrate_enable()? Along with the KVFREE_BULK_MAX_ENTR check? Thanx, Paul