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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EF995C001E0 for ; Wed, 26 Jul 2023 09:33:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231636AbjGZJd3 (ORCPT ); Wed, 26 Jul 2023 05:33:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50926 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233736AbjGZJdI (ORCPT ); Wed, 26 Jul 2023 05:33:08 -0400 Received: from out-36.mta0.migadu.com (out-36.mta0.migadu.com [IPv6:2001:41d0:1004:224b::24]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E61542122 for ; Wed, 26 Jul 2023 02:31:43 -0700 (PDT) Content-Type: text/plain; charset=us-ascii DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1690363901; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=C9JATXkXZaN7f0jrXVOoaEMLIPnzZ4EB9BwhBVu88/k=; b=BJaw31GHkPVMB5J3bFLk7nCWUXBih3Evut0vs64rGABml16vhPPP+VTCvSpnvcgPtpNdnt lUE24RniQLpCmG5kFgpHNW+1PDmB/qhB9l9vhPT5YKqpl2p08nrVkD19PS1VqQI3/KFSOu LvMc1Yox7zbU5EqPQ+5oAKbHUZVLA0E= MIME-Version: 1.0 Subject: Re: [PATCH v2 43/47] mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred} X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: <20230724094354.90817-44-zhengqi.arch@bytedance.com> Date: Wed, 26 Jul 2023 17:30:53 +0800 Cc: Andrew Morton , david@fromorbit.com, tkhai@ya.ru, Vlastimil Babka , Roman Gushchin , djwong@kernel.org, Christian Brauner , "Paul E. McKenney" , tytso@mit.edu, steven.price@arm.com, cel@kernel.org, Sergey Senozhatsky , yujie.liu@intel.com, Greg KH , LKML , Linux Memory Management List , x86@kernel.org, kvm@vger.kernel.org, xen-devel@lists.xenproject.org, linux-erofs@lists.ozlabs.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-nfs@vger.kernel.org, linux-mtd@lists.infradead.org, rcu@vger.kernel.org, netdev , dri-devel@lists.freedesktop.org, linux-arm-msm@vger.kernel.org, dm-devel@redhat.com, linux-raid@vger.kernel.org, linux-bcache@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <20230724094354.90817-1-zhengqi.arch@bytedance.com> <20230724094354.90817-44-zhengqi.arch@bytedance.com> To: Qi Zheng X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org > On Jul 24, 2023, at 17:43, Qi Zheng = wrote: >=20 > Currently, we maintain two linear arrays per node per memcg, which are > shrinker_info::map and shrinker_info::nr_deferred. And we need to = resize > them when the shrinker_nr_max is exceeded, that is, allocate a new = array, > and then copy the old array to the new array, and finally free the old > array by RCU. >=20 > For shrinker_info::map, we do set_bit() under the RCU lock, so we may = set > the value into the old map which is about to be freed. This may cause = the > value set to be lost. The current solution is not to copy the old map = when > resizing, but to set all the corresponding bits in the new map to 1. = This > solves the data loss problem, but bring the overhead of more pointless > loops while doing memcg slab shrink. >=20 > For shrinker_info::nr_deferred, we will only modify it under the read = lock > of shrinker_rwsem, so it will not run concurrently with the resizing. = But > after we make memcg slab shrink lockless, there will be the same data = loss > problem as shrinker_info::map, and we can't work around it like the = map. >=20 > For such resizable arrays, the most straightforward idea is to change = it > to xarray, like we did for list_lru [1]. We need to do xa_store() in = the > list_lru_add()-->set_shrinker_bit(), but this will cause memory > allocation, and the list_lru_add() doesn't accept failure. A possible > solution is to pre-allocate, but the location of pre-allocation is not > well determined. >=20 > Therefore, this commit chooses to introduce a secondary array for > shrinker_info::{map, nr_deferred}, so that we only need to copy this > secondary array every time the size is resized. Then even if we get = the > old secondary array under the RCU lock, the found map and nr_deferred = are > also true, so no data is lost. >=20 > [1]. = https://lore.kernel.org/all/20220228122126.37293-13-songmuchun@bytedance.c= om/ >=20 > Signed-off-by: Qi Zheng Reviewed-by: Muchun Song