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=-2.6 required=3.0 tests=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 4C75EC2D0F3 for ; Wed, 1 Apr 2020 18:26:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1FC3A20BED for ; Wed, 1 Apr 2020 18:26:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585765579; bh=g5rW8mNTdi3VagM9mgwi86mVK6L7YuGisX9m9WlMVeM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=QUKP9Wm4O+fZsh/Zy8nj/KQD109IiHOOdfXQvirucEgzgeNfEmqzpdL5fpRfQImor a+nGGlgGpFw8YjGUwF2WnJHTUNMZtTvzWkCcYAkYW4NFUfNoc8NPuNZnqkAtgNh5Kj +XUfVMcnFLYfRBn0qvU+u7kFQx0Bw3w6dJEX8kUw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732938AbgDAS0R (ORCPT ); Wed, 1 Apr 2020 14:26:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:48652 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732316AbgDAS0R (ORCPT ); Wed, 1 Apr 2020 14:26:17 -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 3126720787; Wed, 1 Apr 2020 18:26:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585765576; bh=g5rW8mNTdi3VagM9mgwi86mVK6L7YuGisX9m9WlMVeM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=IClBs5dYySQ82cb01rO7lAJp3sYnbRIgHn0i5zmMl60flqGa447whUZtMaHtPgVWO w2ZHoy+n0AkoNDyFdo8QpqFtYndJ7vkGQ7aNcVfrwyh8bwDPafD9gDGQS9XlRDe+gV pDf2eUunVwpK5YZp6RttIN40h/4WqQrtOcNkRhsE= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id BA12235226B3; Wed, 1 Apr 2020 11:26:15 -0700 (PDT) Date: Wed, 1 Apr 2020 11:26:15 -0700 From: "Paul E. McKenney" To: Uladzislau Rezki Cc: Joel Fernandes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, willy@infradead.org, peterz@infradead.org, neilb@suse.com, vbabka@suse.cz, mgorman@suse.de, Andrew Morton , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , Steven Rostedt Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern Message-ID: <20200401182615.GE19865@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200331131628.153118-1-joel@joelfernandes.org> <20200331140433.GA26498@pc636> <20200331150911.GC236678@google.com> <20200331160119.GA27614@pc636> <20200331183000.GD236678@google.com> <20200401122550.GA32593@pc636> <20200401134745.GV19865@paulmck-ThinkPad-P72> <20200401181601.GA4042@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200401181601.GA4042@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 Wed, Apr 01, 2020 at 08:16:01PM +0200, Uladzislau Rezki wrote: > > > > > > > > Right. Per discussion with Paul, we discussed that it is better if we > > > > pre-allocate N number of array blocks per-CPU and use it for the cache. > > > > Default for N being 1 and tunable with a boot parameter. I agree with this. > > > > > > > As discussed before, we can make use of memory pool API for such > > > purpose. But i am not sure if it should be one pool per CPU or > > > one pool per NR_CPUS, that would contain NR_CPUS * N pre-allocated > > > blocks. > > > > There are advantages and disadvantages either way. The advantage of the > > per-CPU pool is that you don't have to worry about something like lock > > contention causing even more pain during an OOM event. One potential > > problem wtih the per-CPU pool can happen when callbacks are offloaded, > > in which case the CPUs needing the memory might never be getting it, > > because in the offloaded case (RCU_NOCB_CPU=y) the CPU posting callbacks > > might never be invoking them. > > > > But from what I know now, systems built with CONFIG_RCU_NOCB_CPU=y > > either don't have heavy callback loads (HPC systems) or are carefully > > configured (real-time systems). Plus large systems would probably end > > up needing something pretty close to a slab allocator to keep from dying > > from lock contention, and it is hard to justify that level of complexity > > at this point. > > > > Or is there some way to mark a specific slab allocator instance as being > > able to keep some amount of memory no matter what the OOM conditions are? > > If not, the current per-CPU pre-allocated cache is a better choice in the > > near term. > > > As for mempool API: > > mempool_alloc() just tries to make regular allocation taking into > account passed gfp_t bitmask. If it fails due to memory pressure, > it uses reserved preallocated pool that consists of number of > desirable elements(preallocated when a pool is created). > > mempoll_free() returns an element to to pool, if it detects that > current reserved elements are lower then minimum allowed elements, > it will add an element to reserved pool, i.e. refill it. Otherwise > just call kfree() or whatever we define as "element-freeing function." Unless I am missing something, mempool_alloc() acquires a per-mempool lock on each invocation under OOM conditions. For our purposes, this is essentially a global lock. This will not be at all acceptable on a large system. Thanx, Paul > > If not, the current per-CPU pre-allocated cache is a better choice in the > > near term. > > > OK. I see your point. > > Thank you for your comments and view :) > > -- > Vlad Rezki