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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, 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 32924C433E6 for ; Wed, 10 Mar 2021 18:51:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ED9CE64DF4 for ; Wed, 10 Mar 2021 18:51:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233439AbhCJSuz (ORCPT ); Wed, 10 Mar 2021 13:50:55 -0500 Received: from mail.kernel.org ([198.145.29.99]:41922 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233425AbhCJSup (ORCPT ); Wed, 10 Mar 2021 13:50:45 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 3075764DFD; Wed, 10 Mar 2021 18:50:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1615402245; bh=7EMDjMedyrLG3zrgNV64e8xd6a+Bn6iIf+l9PAZ1tGE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=nmWbZHfB9JtBuSkosE6pT/lAzdo4BKucpd7KEi+VVGn/LYd6RN9xO/ngy6DC3ed/K RHAHp91+DHRvhjCfzRt298Q3XxxqcNrFZNA7IrfJFYOuf+AvT2a3qy3b47poe93CRH 7/FP6KhfHm+SjguqTAcD1BvPLPnPLyJ1SBzNWl0Y= Date: Wed, 10 Mar 2021 19:50:43 +0100 From: Greg Kroah-Hartman To: Linus Torvalds Cc: Linux Kernel Mailing List , stable , Jann Horn , David Rientjes , Joonsoo Kim , Christoph Lameter , Pekka Enberg , Andrew Morton Subject: Re: [PATCH 4.14 27/50] mm, slub: consider rest of partial list if acquire_slab() fails Message-ID: References: <20210122135735.176469491@linuxfoundation.org> <20210122135736.291270624@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 10, 2021 at 10:43:33AM -0800, Linus Torvalds wrote: > Just a note to the stable tree: this commit has been reverted > upstream, because it causes a huge performance drop (admittedly on a > load and setup that may not be all that relevant to most people). > > It was applied to 4.4, 4.9 and 4.12, because the commit it was marked > as "fixing" is from 2012, but it turns out that the early exit from > the loop in that commit was very much intentional, and very much shows > up on scalability benchmarks. > > I don't think this is likely to be a big deal for the stable kernels - > we're basically talking tuning for special cases, and while it is > reverted in my tree now, the "correct" thing to do is likely to be a > bit more flexible than either "exit loop immediately" or "loop for as > long as we have contention". > > In practice, most machines probably won't see either case - or it will > at least be rare enough that you can't tell. > > The machine that reports a huge performance drop was a multi-socket > machine under fairly extreme conditions, and these contention issues > are often close to exponential - a smaller machine (or a slighly less > extreme load) would never see the issue at all either way. > > See > > https://lore.kernel.org/lkml/20210301080404.GF12822@xsang-OptiPlex-9020/ > > for details if you care. I don't think this has to necessarily be > undone in the stable trees, this email is more of an incidental note > just as a heads-up. Thanks for the details, I'll look into reverting it in a future stable release. greg k-h