From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from dggsgout11.his.huawei.com (dggsgout11.his.huawei.com [45.249.212.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5207733F8B1; Mon, 11 May 2026 09:05:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.249.212.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778490329; cv=none; b=U4ScQFTbyQpWM0DFAJWkyUytM1q5dKPb4jb57C6tyhUf8E/mTYcJLwS/VETTNCA4ca8C6Tg2YUznjOGw35fOzj9n+7LrhdPfdZw9qV2i/Ilo32E9J+TLkf61ljBheJ0ba72yNprF99pXwcFwvS5jrbw+qTllUZy0VFejbER2J/g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778490329; c=relaxed/simple; bh=srk5mZG7aTdHgcZkrQCyoDZYyMrqXbpKqPz+lXoIZSQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MuUkK3/bLYgV4OzeFsmLhLiKOQYOIOo1Oe4HYD8cZJ4JKTORxWHqGTCp4KUAn7dtrweEX5caO7YnCoi7kEc58WN5oYWVvSxH2Db/2VlbxMFEtFMrzpJ54AnmSf2cj8gfgRQR2ia72kyisM0Lwqr1bmR+cGSkJ+Q31Sz0M6lEdQU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=huaweicloud.com; spf=pass smtp.mailfrom=huaweicloud.com; arc=none smtp.client-ip=45.249.212.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=huaweicloud.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huaweicloud.com Received: from mail.maildlp.com (unknown [172.19.163.198]) by dggsgout11.his.huawei.com (SkyGuard) with ESMTPS id 4gDYhL1dVSzYQv2f; Mon, 11 May 2026 17:04:50 +0800 (CST) Received: from mail02.huawei.com (unknown [10.116.40.252]) by mail.maildlp.com (Postfix) with ESMTP id 047C840576; Mon, 11 May 2026 17:05:19 +0800 (CST) Received: from [10.67.111.176] (unknown [10.67.111.176]) by APP3 (Coremail) with SMTP id _Ch0CgDHJC3NmwFqxEJ0Bw--.56076S2; Mon, 11 May 2026 17:05:18 +0800 (CST) Message-ID: <22d91fdc-db54-4bcf-bc5a-2a496cc43057@huaweicloud.com> Date: Mon, 11 May 2026 17:05:17 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] cgroup: Keep favordynmods enabled once per-threadgroup rwsem is active To: Guopeng Zhang , Tejun Heo , Johannes Weiner , =?UTF-8?Q?Michal_Koutn=C3=BD?= Cc: Yi Tao , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260511081607.83490-1-zhangguopeng@kylinos.cn> Content-Language: en-US From: Chen Ridong In-Reply-To: <20260511081607.83490-1-zhangguopeng@kylinos.cn> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-CM-TRANSID:_Ch0CgDHJC3NmwFqxEJ0Bw--.56076S2 X-Coremail-Antispam: 1UD129KBjvJXoWxAw17tFWxKr4DKFWDKr4Uurg_yoW5ZrWfpa nrAryftws8KFn0yas7ta40qF18Jr4FqFW7JFWDKw18Aw13GrsYqr4Ik3WUXF1jvFnrGFW7 JwnIyr4kCF4qyrJanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDU0xBIdaVrnRJUUUyGb4IE77IF4wAFF20E14v26r4j6ryUM7CY07I20VC2zVCF04k2 6cxKx2IYs7xG6rWj6s0DM7CIcVAFz4kK6r1j6r18M28lY4IEw2IIxxk0rwA2F7IY1VAKz4 vEj48ve4kI8wA2z4x0Y4vE2Ix0cI8IcVAFwI0_tr0E3s1l84ACjcxK6xIIjxv20xvEc7Cj xVAFwI0_Gr1j6F4UJwA2z4x0Y4vEx4A2jsIE14v26rxl6s0DM28EF7xvwVC2z280aVCY1x 0267AKxVW0oVCq3wAS0I0E0xvYzxvE52x082IY62kv0487Mc02F40EFcxC0VAKzVAqx4xG 6I80ewAv7VC0I7IYx2IY67AKxVWUJVWUGwAv7VC2z280aVAFwI0_Jr0_Gr1lOx8S6xCaFV Cjc4AY6r1j6r4UM4x0Y48IcVAKI48JMxkF7I0En4kS14v26r126r1DMxAIw28IcxkI7VAK I48JMxC20s026xCaFVCjc4AY6r1j6r4UMI8I3I0E5I8CrVAFwI0_Jr0_Jr4lx2IqxVCjr7 xvwVAFwI0_JrI_JrWlx4CE17CEb7AF67AKxVWUAVWUtwCIc40Y0x0EwIxGrwCI42IY6xII jxv20xvE14v26r1j6r1xMIIF0xvE2Ix0cI8IcVCY1x0267AKxVWUJVW8JwCI42IY6xAIw2 0EY4v20xvaj40_Jr0_JF4lIxAIcVC2z280aVAFwI0_Jr0_Gr1lIxAIcVC2z280aVCY1x02 67AKxVW8JVW8JrUvcSsGvfC2KfnxnUUI43ZEXa7IU1veHDUUUUU== X-CM-SenderInfo: hfkh02xlgr0w46kxt4xhlfz01xgou0bp/ On 2026/5/11 16:16, Guopeng Zhang wrote: > cgroup_enable_per_threadgroup_rwsem is a one-way switch. Once it is > enabled, cgroup.procs writes use the per-threadgroup rwsem and > cgroup_threadgroup_change_begin()/end() use the same global state to > decide whether to take and release the per-threadgroup rwsem. > > The disable path warned that the per-threadgroup rwsem mechanism could not > be disabled but still called rcu_sync_exit() and cleared > CGRP_ROOT_FAVOR_DYNMODS. That partially disabled favordynmods while the > global per-threadgroup rwsem mode remained enabled: cgroup.procs writes > would continue to use the per-threadgroup rwsem, while > cgroup_threadgroup_change_begin()/end() could observe the exited rcu_sync > state. The root would also no longer report favordynmods. > > Make the transition match the documented one-way semantics. Call > rcu_sync_enter() only for the first favordynmods enable, and make later > disable attempts a no-op after warning once the per-threadgroup rwsem mode > has been enabled. > > Fixes: 0568f89d4fb8 ("cgroup: replace global percpu_rwsem with per threadgroup resem when writing to cgroup.procs") > Signed-off-by: Guopeng Zhang > --- > Manual AB test: > > Before this patch: > enable favordynmods: > cgroup2 opts: rw,relatime,favordynmods > disable attempt: > cgroup2 opts: rw,relatime > dmesg: > cgroup: cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled > > After this patch: > enable favordynmods: > cgroup2 opts: rw,relatime,favordynmods > disable attempt: > cgroup2 opts: rw,relatime,favordynmods > dmesg: > cgroup: cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled > > kernel/cgroup/cgroup.c | 11 +++++------ > 1 file changed, 5 insertions(+), 6 deletions(-) > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c > index 6152add0c5eb..fd10fb5b3598 100644 > --- a/kernel/cgroup/cgroup.c > +++ b/kernel/cgroup/cgroup.c > @@ -1297,14 +1297,13 @@ void cgroup_favor_dynmods(struct cgroup_root *root, bool favor) > */ > percpu_down_write(&cgroup_threadgroup_rwsem); > if (favor && !favoring) { > - cgroup_enable_per_threadgroup_rwsem = true; > - rcu_sync_enter(&cgroup_threadgroup_rwsem.rss); > + if (!cgroup_enable_per_threadgroup_rwsem) { Is this branch redundant? I think if (favor && !favoring) alone should suffice — or can the outer condition be true twice (i.e., can this block be entered multiple times)? > + cgroup_enable_per_threadgroup_rwsem = true; > + rcu_sync_enter(&cgroup_threadgroup_rwsem.rss); > + } > root->flags |= CGRP_ROOT_FAVOR_DYNMODS; > } else if (!favor && favoring) { > - if (cgroup_enable_per_threadgroup_rwsem) > - pr_warn_once("cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled\n"); > - rcu_sync_exit(&cgroup_threadgroup_rwsem.rss); > - root->flags &= ~CGRP_ROOT_FAVOR_DYNMODS; > + pr_warn_once("cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled\n"); > } > percpu_up_write(&cgroup_threadgroup_rwsem); > } -- Best regards, Ridong