From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 1D0446B24C1 for ; Wed, 22 Aug 2018 10:12:17 -0400 (EDT) Received: by mail-ed1-f72.google.com with SMTP id c16-v6so1000062edc.21 for ; Wed, 22 Aug 2018 07:12:17 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id d61-v6si2312781edd.124.2018.08.22.07.12.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 22 Aug 2018 07:12:15 -0700 (PDT) Date: Wed, 22 Aug 2018 16:12:13 +0200 From: Michal Hocko Subject: Re: [PATCH v2 1/3] mm: rework memcg kernel stack accounting Message-ID: <20180822141213.GO29735@dhcp22.suse.cz> References: <20180821213559.14694-1-guro@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180821213559.14694-1-guro@fb.com> Sender: owner-linux-mm@kvack.org List-ID: To: Roman Gushchin Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Johannes Weiner , Andy Lutomirski , Konstantin Khlebnikov , Tejun Heo , Shakeel Butt On Tue 21-08-18 14:35:57, Roman Gushchin wrote: > If CONFIG_VMAP_STACK is set, kernel stacks are allocated > using __vmalloc_node_range() with __GFP_ACCOUNT. So kernel > stack pages are charged against corresponding memory cgroups > on allocation and uncharged on releasing them. > > The problem is that we do cache kernel stacks in small > per-cpu caches and do reuse them for new tasks, which can > belong to different memory cgroups. > > Each stack page still holds a reference to the original cgroup, > so the cgroup can't be released until the vmap area is released. > > To make this happen we need more than two subsequent exits > without forks in between on the current cpu, which makes it > very unlikely to happen. As a result, I saw a significant number > of dying cgroups (in theory, up to 2 * number_of_cpu + > number_of_tasks), which can't be released even by significant > memory pressure. > > As a cgroup structure can take a significant amount of memory > (first of all, per-cpu data like memcg statistics), it leads > to a noticeable waste of memory. > > Signed-off-by: Roman Gushchin > Cc: Johannes Weiner > Cc: Michal Hocko > Cc: Andy Lutomirski > Cc: Konstantin Khlebnikov > Cc: Tejun Heo > Cc: Shakeel Butt Looks good to me. Two nits below. I am not sure stable tree backport is really needed but it would be nice to put Fixes: ac496bf48d97 ("fork: Optimize task creation by caching two thread stacks per CPU if CONFIG_VMAP_STACK=y") Acked-by: Michal Hocko > @@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node) > static inline void free_thread_stack(struct task_struct *tsk) > { > #ifdef CONFIG_VMAP_STACK > - if (task_stack_vm_area(tsk)) { > + struct vm_struct *vm = task_stack_vm_area(tsk); > + > + if (vm) { > int i; > > + for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) { > + mod_memcg_page_state(vm->pages[i], > + MEMCG_KERNEL_STACK_KB, > + -(int)(PAGE_SIZE / 1024)); > + > + memcg_kmem_uncharge(vm->pages[i], > + compound_order(vm->pages[i])); when do we have order > 0 here? Also I was wondering how come this doesn't blow up on partially charged stacks but both mod_memcg_page_state and memcg_kmem_uncharge check for page->mem_cgroup so this is safe. Maybe a comment would save people from scratching their heads. -- Michal Hocko SUSE Labs