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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 358F8C282C8 for ; Mon, 28 Jan 2019 20:10:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0F919214DA for ; Mon, 28 Jan 2019 20:10:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727430AbfA1UKb (ORCPT ); Mon, 28 Jan 2019 15:10:31 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:47326 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726661AbfA1UKb (ORCPT ); Mon, 28 Jan 2019 15:10:31 -0500 Received: from akpm3.svl.corp.google.com (unknown [104.133.8.65]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id E475623FA; Mon, 28 Jan 2019 20:10:29 +0000 (UTC) Date: Mon, 28 Jan 2019 12:10:28 -0800 From: Andrew Morton To: Rik van Riel Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@fb.com, Johannes Weiner , Chris Mason , Roman Gushchin , Michal Hocko Subject: Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small slabs Message-Id: <20190128121028.8ef4c19dd3fd1d60d2e3284c@linux-foundation.org> In-Reply-To: <8ddf2ea674711f373062f4e056dd14fb81c5a2fe.camel@surriel.com> References: <20190128143535.7767c397@imladris.surriel.com> <20190128115424.df3f4647023e9e43e75afe67@linux-foundation.org> <8ddf2ea674711f373062f4e056dd14fb81c5a2fe.camel@surriel.com> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 28 Jan 2019 15:03:28 -0500 Rik van Riel wrote: > On Mon, 2019-01-28 at 11:54 -0800, Andrew Morton wrote: > > On Mon, 28 Jan 2019 14:35:35 -0500 Rik van Riel > > wrote: > > > > > /* > > > * Make sure we apply some minimal pressure on default priority > > > - * even on small cgroups. Stale objects are not only consuming > > > memory > > > + * even on small cgroups, by accumulating pressure across > > > multiple > > > + * slab shrinker runs. Stale objects are not only consuming > > > memory > > > * by themselves, but can also hold a reference to a dying > > > cgroup, > > > * preventing it from being reclaimed. A dying cgroup with all > > > * corresponding structures like per-cpu stats and kmem caches > > > * can be really big, so it may lead to a significant waste of > > > memory. > > > */ > > > - delta = max_t(unsigned long long, delta, min(freeable, > > > batch_size)); > > > + if (!delta) { > > > + shrinker->small_scan += freeable; > > > + > > > + delta = shrinker->small_scan >> priority; > > > + shrinker->small_scan -= delta << priority; > > > + > > > + delta *= 4; > > > + do_div(delta, shrinker->seeks); > > > > What prevents shrinker->small_scan from over- or underflowing over > > time? > > We only go into this code path if > delta >> DEF_PRIORITY is zero. > > That is, freeable is smaller than 4096. > I'm still not understanding. If `freeable' always has a value of (say) 1, we'll eventually overflow shrinker->small_scan? Or at least, it's unobvious why this cannot happen.