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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 79404C433F5 for ; Thu, 2 Sep 2021 21:59:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 66D1160E8B for ; Thu, 2 Sep 2021 21:59:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347843AbhIBWAx (ORCPT ); Thu, 2 Sep 2021 18:00:53 -0400 Received: from mail.kernel.org ([198.145.29.99]:56956 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1347763AbhIBWAx (ORCPT ); Thu, 2 Sep 2021 18:00:53 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id F32E060F21; Thu, 2 Sep 2021 21:59:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1630619994; bh=ViDStTCz7Z8TyQQO4iJiePVQ6NLIUsl8TyTWolpn1xU=; h=Date:From:To:Subject:In-Reply-To:From; b=aWL2owpTVws7zAe8PuyNMt26GpQzSOdn1mgviA9iMBDSDMFw9kLo5r1xB9dEL3zMy n9YKE5bD3L4JipD9Dmfr0mzeoWX2zT6LJlGTR6erxgpHliLG1lu648Y4O3DqOtNL9T GB/fCkXrW0TdZBhwN4JnVe+oTH1++gFnkkm+h3AQ= Date: Thu, 02 Sep 2021 14:59:53 -0700 From: Andrew Morton To: akpm@linux-foundation.org, chris@chrisdown.name, linux-mm@kvack.org, mhocko@kernel.org, mm-commits@vger.kernel.org, songmuchun@bytedance.com, torvalds@linux-foundation.org, vbabka@suse.cz, wangkefeng.wang@huawei.com, willy@infradead.org, zangchunxin@bytedance.com Subject: [patch 187/212] mm, vmscan: guarantee drop_slab_node() termination Message-ID: <20210902215953.otpQ1y0ro%akpm@linux-foundation.org> In-Reply-To: <20210902144820.78957dff93d7bea620d55a89@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Vlastimil Babka Subject: mm, vmscan: guarantee drop_slab_node() termination drop_slab_node() is called as part of echo 2>/proc/sys/vm/drop_caches operation. It iterates over all memcgs and calls shrink_slab() which in turn iterates over all slab shrinkers. Freed objects are counted and as long as the total number of freed objects from all memcgs and shrinkers is higher than 10, drop_slab_node() loops for another full memcgs*shrinkers iteration. This arbitrary constant threshold of 10 can result in effectively an infinite loop on a system with large number of memcgs and/or parallel activity that allocates new objects. This has been reported previously by Chunxin Zang [1] and recently by our customer. The previous report [1] has resulted in commit 069c411de40a ("mm/vmscan: fix infinite loop in drop_slab_node") which added a check for signals allowing the user to terminate the command writing to drop_caches. At the time it was also considered to make the threshold grow with each iteration to guarantee termination, but such patch hasn't been formally proposed yet. This patch implements the dynamically growing threshold. At first iteration it's enough to free one object to continue, and this threshold effectively doubles with each iteration. Our customer's feedback was positive. There is always a risk that this change will result on some system in a previously terminating drop_caches operation to terminate sooner and free fewer objects. Ideally the semantics would guarantee freeing all freeable objects that existed at the moment of starting the operation, while not looping forever for newly allocated objects, but that's not feasible to track. In the less ideal solution based on thresholds, arguably the termination guarantee is more important than the exhaustiveness guarantee. If there are reports of large regression wrt being exhaustive, we can tune how fast the threshold grows. [1] https://lore.kernel.org/lkml/20200909152047.27905-1-zangchunxin@bytedance.com/T/#u [vbabka@suse.cz: avoid undefined shift behaviour] Link: https://lkml.kernel.org/r/2f034e6f-a753-550a-f374-e4e23899d3d5@suse.cz Link: https://lkml.kernel.org/r/20210818152239.25502-1-vbabka@suse.cz Signed-off-by: Vlastimil Babka Reported-by: Chunxin Zang Cc: Muchun Song Cc: Chris Down Cc: Michal Hocko Cc: Matthew Wilcox Cc: Vlastimil Babka Cc: Kefeng Wang Signed-off-by: Andrew Morton --- mm/vmscan.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/mm/vmscan.c~mm-vmscan-guarantee-drop_slab_node-termination +++ a/mm/vmscan.c @@ -939,6 +939,7 @@ out: void drop_slab_node(int nid) { unsigned long freed; + int shift = 0; do { struct mem_cgroup *memcg = NULL; @@ -951,7 +952,7 @@ void drop_slab_node(int nid) do { freed += shrink_slab(GFP_KERNEL, nid, memcg, 0); } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); - } while (freed > 10); + } while ((freed >> shift++) > 1); } void drop_slab(void) _