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=-3.9 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 B77C2C43466 for ; Mon, 21 Sep 2020 07:47:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6B4552085B for ; Mon, 21 Sep 2020 07:47:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="f9awOxBI" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726441AbgIUHrU (ORCPT ); Mon, 21 Sep 2020 03:47:20 -0400 Received: from mx2.suse.de ([195.135.220.15]:47510 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726236AbgIUHrT (ORCPT ); Mon, 21 Sep 2020 03:47:19 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1600674438; 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=9i0kIJxHfcUjwHDlDqQFWcrJLdfDzO+qklY0v++fUcM=; b=f9awOxBIS+NYd8MBmsXro8e9poIUcVJyAN6gV8CcgrcuElzeDdmJp+h3+N/vXQRnY1kX3m bhL04Z1Wbx/Qh730kRMUS8CZtKUMkyQg4PvPPTgWg0D//j+KNshfsl118IrzsbQMtK73PQ SFNVYue3YGwLeGtMFOXSqsb4crubwow= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 08BE8AFC0; Mon, 21 Sep 2020 07:47:54 +0000 (UTC) Date: Mon, 21 Sep 2020 09:47:16 +0200 From: Michal Hocko To: "Uladzislau Rezki (Sony)" Cc: LKML , RCU , linux-mm@kvack.org, Andrew Morton , "Paul E . McKenney" , Peter Zijlstra , Vlastimil Babka , Thomas Gleixner , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20200921074716.GC12990@dhcp22.suse.cz> References: <20200918194817.48921-1-urezki@gmail.com> <20200918194817.48921-3-urezki@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200918194817.48921-3-urezki@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 18-09-20 21:48:15, Uladzislau Rezki (Sony) wrote: [...] > Proposal > ======== > Introduce a lock-free function that obtain a page from the per-cpu-lists > on current CPU. It returns NULL rather than acquiring any non-raw spinlock. I was not happy about this solution when we have discussed this last time and I have to say I am still not happy. This is exposing an internal allocator optimization and allows a hard to estimate consumption of pcp free pages. IIUC this run on pcp cache can be controled directly from userspace (close(open) loop IIRC) which makes it even bigger no-no. I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@nanos.tec.linutronix.de that this optimization is not aiming at reasonable workloads. Really, go with pre-allocated buffer and fallback to whatever slow path you have already. Exposing more internals of the allocator is not going to do any good for long term maintainability. -- Michal Hocko SUSE Labs