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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, 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 38B35C47257 for ; Mon, 4 May 2020 18:07:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 15CBE206B8 for ; Mon, 4 May 2020 18:07:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588615664; bh=qSSPanwBoB9/JQpMTRvAt64HeB1Nngc7+wt849hUqt8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=jH36CEADeCe+jh+xyT857Zw3lPpVxuNuCvUFJCIgdUtB7IMfApq0qgHa06FKm9TYe cLhtIuyUoIT0X4MO1cngZLNRVA9M/ZymTUXyhYBLYCvb9LvwFozgtzkRnbI2FCzD7n felpLpIishI62/AURowIlu/m/bQ1A5IYiSGhVOrg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732179AbgEDSHn (ORCPT ); Mon, 4 May 2020 14:07:43 -0400 Received: from mail.kernel.org ([198.145.29.99]:38524 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731094AbgEDSHi (ORCPT ); Mon, 4 May 2020 14:07:38 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 620EA206B8; Mon, 4 May 2020 18:07:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588615657; bh=qSSPanwBoB9/JQpMTRvAt64HeB1Nngc7+wt849hUqt8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=gOJg6N0Wp5t9LhWqGYLQdlvrM4KGIMDVXHt5MuhbymoZ/3c9ufbSMdZ4v7ElfUGP2 bk4lSTe5mv4yys6Jv2pdJpujGo7i2Mv6UTmnts5leniCOTcyRKUWKg+dq0yhHyBISZ AGUsWGxPfyMCmb4hywnL0NSJq1KaBwv2pWOsU0ww= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 437E535226F4; Mon, 4 May 2020 11:07:37 -0700 (PDT) Date: Mon, 4 May 2020 11:07:37 -0700 From: "Paul E. McKenney" To: Uladzislau Rezki Cc: LKML , linux-mm@kvack.org, Andrew Morton , "Theodore Y . Ts'o" , Matthew Wilcox , Joel Fernandes , RCU , Oleksiy Avramchenko Subject: Re: [PATCH 09/24] rcu/tree: cache specified number of objects Message-ID: <20200504180737.GT2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200428205903.61704-1-urezki@gmail.com> <20200428205903.61704-10-urezki@gmail.com> <20200501212749.GD7560@paulmck-ThinkPad-P72> <20200504124323.GA17577@pc636> <20200504152437.GK2869@paulmck-ThinkPad-P72> <20200504174822.GA20446@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200504174822.GA20446@pc636> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 04, 2020 at 07:48:22PM +0200, Uladzislau Rezki wrote: > On Mon, May 04, 2020 at 08:24:37AM -0700, Paul E. McKenney wrote: > > On Mon, May 04, 2020 at 02:43:23PM +0200, Uladzislau Rezki wrote: > > > On Fri, May 01, 2020 at 02:27:49PM -0700, Paul E. McKenney wrote: > > > > On Tue, Apr 28, 2020 at 10:58:48PM +0200, Uladzislau Rezki (Sony) wrote: > > > > > Cache some extra objects per-CPU. During reclaim process > > > > > some pages are cached instead of releasing by linking them > > > > > into the list. Such approach provides O(1) access time to > > > > > the cache. > > > > > > > > > > That reduces number of requests to the page allocator, also > > > > > that makes it more helpful if a low memory condition occurs. > > > > > > > > > > A parameter reflecting the minimum allowed pages to be > > > > > cached per one CPU is propagated via sysfs, it is read > > > > > only, the name is "rcu_min_cached_objs". > > > > > > > > > > Signed-off-by: Uladzislau Rezki (Sony) > > > > > --- > > > > > kernel/rcu/tree.c | 64 ++++++++++++++++++++++++++++++++++++++++++++--- > > > > > 1 file changed, 60 insertions(+), 4 deletions(-) > > > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > > index 89e9ca3f4e3e..d8975819b1c9 100644 > > > > > --- a/kernel/rcu/tree.c > > > > > +++ b/kernel/rcu/tree.c > > > > > @@ -178,6 +178,14 @@ module_param(gp_init_delay, int, 0444); > > > > > static int gp_cleanup_delay; > > > > > module_param(gp_cleanup_delay, int, 0444); > > > > > > > > > > +/* > > > > > + * This rcu parameter is read-only, but can be write also. > > > > > > > > You mean that although the parameter is read-only, you see no reason > > > > why it could not be converted to writeable? > > > > > > > I added just a note. If it is writable, then we can change the size of the > > > per-CPU cache dynamically, i.e. "echo 5 > /sys/.../rcu_min_cached_objs" > > > would cache 5 pages. But i do not have a strong opinion if it should be > > > writable. > > > > > > > If it was writeable, and a given CPU had the maximum numbr of cached > > > > objects, the rcu_min_cached_objs value was decreased, but that CPU never > > > > saw another kfree_rcu(), would the number of cached objects change? > > > > > > > No. It works the way: unqueue the page from cache in the kfree_rcu(), > > > whereas "rcu work" will put it back if number of objects < rcu_min_cached_objs, > > > if >= will free the page. > > > > Just to make sure I understand... If someone writes a smaller number to > > the sysfs variable, the per-CPU caches will be decreased at that point, > > immediately during that sysfs write? Or are you saying something else? > > > This patch defines it as read-only. It defines the minimum threshold that > controls number of elements in the per-CPU cache. If we decide to make it > write also, then we will have full of freedom how to define its behavior, > i.e. it is not defined because it is read only. And runtime-read-only sounds like an excellent state for it. > > > > Presumably the list can also be accessed without holding this lock, > > > > because otherwise we shouldn't need llist... > > > > > > > Hm... We increase the number of elements in cache, therefore it is not > > > lockless. From the other hand i used llist_head to maintain the cache > > > because it is single linked list, we do not need "*prev" link. Also > > > we do not need to init the list. > > > > > > But i can change it to list_head. Please let me know if i need :) > > > > Hmmm... Maybe it is time for a non-atomic singly linked list? In the RCU > > callback processing, the operations were open-coded, but they have been > > pushed into include/linux/rcu_segcblist.h and kernel/rcu/rcu_segcblist.*. > > > > Maybe some non-atomic/protected/whatever macros in the llist.h file? > > Or maybe just open-code the singly linked list? (Probably not the > > best choice, though.) Add comments stating that the atomic properties > > of the llist functions aren't neded? Something else? > > > In order to keep it simple i can replace llist_head by the list_head? Fine by me! > > The comments would be a good start. Just to take pity on people seeing > > the potential for concurrency and wondering how the concurrent accesses > > actually happen. ;-) > > > Sounds like you are kidding me :) "Only those who have gone too far can possibly tell you how far you can go!" ;-) Thanx, Paul