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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 54FE2C43331 for ; Thu, 7 Nov 2019 17:02:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 239062166E for ; Thu, 7 Nov 2019 17:02:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573146124; bh=W+CKAoVKOrQ1tCetrkjkS2dgjL5jTJK79aS6yD98cYw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=2b4OeH5HZZzxoM4PASBFp+jtGC8ZvFO33wyU/pegyCUT28UyHQcdYVqiDYuiC+QNp UEW8bkN1bndMzQa61R6SjM/LgicAATc5DFKqiODixwTRRBBKfOCUaoNv+YIoJsTVrf //VdIryfvR3CPb2G8dUiSbw1dONzqno/0iWdMhb4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726877AbfKGRCD (ORCPT ); Thu, 7 Nov 2019 12:02:03 -0500 Received: from mx2.suse.de ([195.135.220.15]:47682 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726231AbfKGRCD (ORCPT ); Thu, 7 Nov 2019 12:02:03 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 92CBAAB9D; Thu, 7 Nov 2019 17:02:01 +0000 (UTC) Date: Thu, 7 Nov 2019 18:02:00 +0100 From: Michal Hocko To: Roman Gushchin Cc: "linux-mm@kvack.org" , Andrew Morton , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team , "stable@vger.kernel.org" , Tejun Heo Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm() Message-ID: <20191107170200.GX8314@dhcp22.suse.cz> References: <20191106225131.3543616-1-guro@fb.com> <20191107122125.GS8314@dhcp22.suse.cz> <20191107164236.GB2919@castle.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191107164236.GB2919@castle.dhcp.thefacebook.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Thu 07-11-19 16:42:41, Roman Gushchin wrote: > On Thu, Nov 07, 2019 at 01:21:25PM +0100, Michal Hocko wrote: > > On Wed 06-11-19 14:51:30, Roman Gushchin wrote: > > > We've encountered a rcu stall in get_mem_cgroup_from_mm(): > > > > > > rcu: INFO: rcu_sched self-detected stall on CPU > > > rcu: 33-....: (21000 ticks this GP) idle=6c6/1/0x4000000000000002 softirq=35441/35441 fqs=5017 > > > (t=21031 jiffies g=324821 q=95837) NMI backtrace for cpu 33 > > > <...> > > > RIP: 0010:get_mem_cgroup_from_mm+0x2f/0x90 > > > <...> > > > __memcg_kmem_charge+0x55/0x140 > > > __alloc_pages_nodemask+0x267/0x320 > > > pipe_write+0x1ad/0x400 > > > new_sync_write+0x127/0x1c0 > > > __kernel_write+0x4f/0xf0 > > > dump_emit+0x91/0xc0 > > > writenote+0xa0/0xc0 > > > elf_core_dump+0x11af/0x1430 > > > do_coredump+0xc65/0xee0 > > > ? unix_stream_sendmsg+0x37d/0x3b0 > > > get_signal+0x132/0x7c0 > > > do_signal+0x36/0x640 > > > ? recalc_sigpending+0x17/0x50 > > > exit_to_usermode_loop+0x61/0xd0 > > > do_syscall_64+0xd4/0x100 > > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > > > The problem is caused by an exiting task which is associated with > > > an offline memcg. > > > > Hmm, how can we have a task in an offline memcg? I thought that any > > existing task will prevent cgroup removal from proceeding. Is this some > > sort of race where the task managed to disassociate from the cgroup > > while there is still a binding to a memcg existing? What am I missing? > > It's an exiting task with the PF_EXITING flag set and it's in their late stages > of life. This is a signal delivery path AFAIU (get_signal) and the coredumping happens before do_exit. My understanding is that that unlinking happens from cgroup_exit. So either I am misreading the backtrace or there is some other way to leave cgroups or there is something more going on. JFTR I am not really disputing the patch but I simply do not understand how the problem really happened. -- Michal Hocko SUSE Labs