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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,URIBL_BLOCKED,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 11274C433DF for ; Mon, 10 Aug 2020 11:56:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E7D442073A for ; Mon, 10 Aug 2020 11:56:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726585AbgHJL4i (ORCPT ); Mon, 10 Aug 2020 07:56:38 -0400 Received: from out30-45.freemail.mail.aliyun.com ([115.124.30.45]:48126 "EHLO out30-45.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726398AbgHJL4i (ORCPT ); Mon, 10 Aug 2020 07:56:38 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R171e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01355;MF=xlpang@linux.alibaba.com;NM=1;PH=DS;RN=11;SR=0;TI=SMTPD_---0U5LoBgT_1597060590; Received: from xunleideMacBook-Pro.local(mailfrom:xlpang@linux.alibaba.com fp:SMTPD_---0U5LoBgT_1597060590) by smtp.aliyun-inc.com(127.0.0.1); Mon, 10 Aug 2020 19:56:31 +0800 Reply-To: xlpang@linux.alibaba.com Subject: Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects To: Pekka Enberg , Christopher Lameter Cc: Vlastimil Babka , Andrew Morton , Wen Yang , Yang Shi , Roman Gushchin , "linux-mm@kvack.org" , LKML , Konstantin Khlebnikov , David Rientjes References: <1593678728-128358-1-git-send-email-xlpang@linux.alibaba.com> From: xunlei Message-ID: Date: Mon, 10 Aug 2020 19:56:30 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020/8/8 上午1:28, Pekka Enberg wrote: > Hi Christopher, > > On Fri, 7 Aug 2020, Pekka Enberg wrote: >>> I think we can just default to the counters. After all, if I >>> understood correctly, we're talking about up to 100 ms time period >>> with IRQs disabled when count_partial() is called. As this is >>> triggerable from user space, that's a performance bug whatever way you >>> look at it. > > On Fri, Aug 7, 2020 at 4:02 PM Christopher Lameter wrote: >> Well yes under extreme conditions and this is only happening for sysfs >> counter retrieval. > > You will likely get some stall even in less extreme conditions, and in > any case, the kernel should not allow user space to trigger such a > stall. > Yes, agree. This problem has been causing lots of trouble to us and other people, and should be fixed. Either my approach or the approach provided by "Vlastimil Babka" [1] is better than doing nothing. [1]: https://lore.kernel.org/linux-mm/158860845968.33385.4165926113074799048.stgit@buzz/ > On Fri, Aug 7, 2020 at 4:02 PM Christopher Lameter wrote: >> There could be other solutions to this. This solution here is penalizing >> evertu hotpath slab allocation for the sake of relatively infrequently >> used counter monitoring. There the possibility of not traversing the list >> ande simply estimating the value based on the number of slab pages >> allocated on that node. > > Why do you consider this to be a fast path? This is all partial list > accounting when we allocate/deallocate a slab, no? Just like > ___slab_alloc() says, I assumed this to be the slow path... What am I > missing? The only hot path is __slab_free(), I've made an extra patch with percpu counter to avoid the potential performance degradation, will send v2 out for review. > > No objections to alternative fixes, of course, but wrapping the > counters under CONFIG_DEBUG seems like just hiding the actual issue... > > - Pekka >