From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roman Gushchin Subject: Re: [PATCH 4/5] mm: rework non-root kmem_cache lifecycle management Date: Thu, 18 Apr 2019 18:14:05 +0000 Message-ID: <20190418181400.GC11008@tower.DHCP.thefacebook.com> References: <20190417215434.25897-1-guro@fb.com> <20190417215434.25897-5-guro@fb.com> <20190418003850.GA13977@tower.DHCP.thefacebook.com> <20190418030729.GA5038@castle> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.com; h=from : to : cc : subject : date : message-id : references : in-reply-to : content-type : content-id : content-transfer-encoding : mime-version; s=facebook; bh=DSUc3ZUilcXF9SyrLXYkcFIObP6rbmpcZ+nA23ILDqs=; b=S4iJSnj7v4nw51t41u2c6Sene2JN/34aQMO5Ss3kZK4gog3bVdWW4/7g9lrBQOvwmcce c0OX/nWyxrJUHs9auO5bxVKRBUaKwCxwu+VAG7o6yqut/z+z4dVZdEvVbnr/Rz4NJ9Fd X5UgtUC7cujdMSNxicZkIi500ng600Sc8NU= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.onmicrosoft.com; s=selector1-fb-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=DSUc3ZUilcXF9SyrLXYkcFIObP6rbmpcZ+nA23ILDqs=; b=PZXzz6WhfCMD6OW0ipbHlcfX+zlTHRvdrwTvcsDFAJUbhpRdnbn0MHv3YTqCImwtJvfgM3LlLj69GyFW7dqa/q3lk90cSQ0g2cMW5XedBJbKVoIB00m0Soh7aFc/N1UD9VHrjzyMl1h30BKLKQIFxyhs2Oy5W5c9ZPYAb1iMjJ8= In-Reply-To: Content-Language: en-US Content-ID: Sender: linux-kernel-owner@vger.kernel.org List-ID: To: Shakeel Butt Cc: Roman Gushchin , Andrew Morton , Linux MM , LKML , Kernel Team , Johannes Weiner , Michal Hocko , Rik van Riel , "david@fromorbit.com" , Christoph Lameter , Pekka Enberg , Vladimir Davydov , Cgroups On Thu, Apr 18, 2019 at 07:05:24AM -0700, Shakeel Butt wrote: > On Wed, Apr 17, 2019 at 8:07 PM Roman Gushchin wrote: > > > > On Wed, Apr 17, 2019 at 06:55:12PM -0700, Shakeel Butt wrote: > > > On Wed, Apr 17, 2019 at 5:39 PM Roman Gushchin wrote: > > > > > > > > On Wed, Apr 17, 2019 at 04:41:01PM -0700, Shakeel Butt wrote: > > > > > On Wed, Apr 17, 2019 at 2:55 PM Roman Gushchin = wrote: > > > > > > > > > > > > This commit makes several important changes in the lifecycle > > > > > > of a non-root kmem_cache, which also affect the lifecycle > > > > > > of a memory cgroup. > > > > > > > > > > > > Currently each charged slab page has a page->mem_cgroup pointer > > > > > > to the memory cgroup and holds a reference to it. > > > > > > Kmem_caches are held by the cgroup. On offlining empty kmem_cac= hes > > > > > > are freed, all other are freed on cgroup release. > > > > > > > > > > No, they are not freed (i.e. destroyed) on offlining, only > > > > > deactivated. All memcg kmem_caches are freed/destroyed on memcg's > > > > > css_free. > > > > > > > > You're right, my bad. I was thinking about the corresponding sysfs = entry > > > > when was writing it. We try to free it from the deactivation path t= oo. > > > > > > > > > > > > > > > > > > > > > So the current scheme can be illustrated as: > > > > > > page->mem_cgroup->kmem_cache. > > > > > > > > > > > > To implement the slab memory reparenting we need to invert the = scheme > > > > > > into: page->kmem_cache->mem_cgroup. > > > > > > > > > > > > Let's make every page to hold a reference to the kmem_cache (we > > > > > > already have a stable pointer), and make kmem_caches to hold a = single > > > > > > reference to the memory cgroup. > > > > > > > > > > What about memcg_kmem_get_cache()? That function assumes that by > > > > > taking reference on memcg, it's kmem_caches will stay. I think yo= u > > > > > need to get reference on the kmem_cache in memcg_kmem_get_cache() > > > > > within the rcu lock where you get the memcg through css_tryget_on= line. > > > > > > > > Yeah, a very good question. > > > > > > > > I believe it's safe because css_tryget_online() guarantees that > > > > the cgroup is online and won't go offline before css_free() in > > > > slab_post_alloc_hook(). I do initialize kmem_cache's refcount to 1 > > > > and drop it on offlining, so it protects the online kmem_cache. > > > > > > > > > > Let's suppose a thread doing a remote charging calls > > > memcg_kmem_get_cache() and gets an empty kmem_cache of the remote > > > memcg having refcnt equal to 1. That thread got a reference on the > > > remote memcg but no reference on the kmem_cache. Let's suppose that > > > thread got stuck in the reclaim and scheduled away. In the meantime > > > that remote memcg got offlined and decremented the refcnt of all of > > > its kmem_caches. The empty kmem_cache which the thread stuck in > > > reclaim have pointer to can get deleted and may be using an already > > > destroyed kmem_cache after coming back from reclaim. > > > > > > I think the above situation is possible unless the thread gets the > > > reference on the kmem_cache in memcg_kmem_get_cache(). > > > > Yes, you're right and I'm writing a nonsense: css_tryget_online() > > can't prevent the cgroup from being offlined. > > >=20 > The reason I knew about that race is because I tried something similar > but for different use-case: >=20 > https://lkml.org/lkml/2018/3/26/472 >=20 > > So, the problem with getting a reference in memcg_kmem_get_cache() > > is that it's an atomic operation on the hot path, something I'd like > > to avoid. > > > > I can make the refcounter percpu, but it'll add some complexity and siz= e > > to the kmem_cache object. Still an option, of course. > > >=20 > I kind of prefer this option. >=20 > > I wonder if we can use rcu_read_lock() instead, and bump the refcounter > > only if we're going into reclaim. > > > > What do you think? >=20 > Should it be just reclaim or anything that can reschedule the current thr= ead? >=20 > I can tell how we resolve the similar issue for our > eager-kmem_cache-deletion use-case. Our solution (hack) works only for > CONFIG_SLAB (we only use SLAB) and non-preemptible kernel. The > underlying motivation was to reduce the overhead of slab reaper of > traversing thousands of empty offlined kmem caches. CONFIG_SLAB > disables interrupts before accessing the per-cpu caches and reenables > the interrupts if it has to fallback to the page allocation. We use > this window to call memcg_kmem_get_cache() and only increment the > refcnt of kmem_cache if going to the fallback. Thus no need to do > atomic operation on the hot path. >=20 > Anyways, I think having percpu refcounter for each memcg kmem_cache is > not that costy for CONFIG_MEMCG_KMEM users and to me that seems like > the most simple solution. >=20 > Shakeel Ok, sounds like a percpu refcounter is the best option. I'll try this approach in v2. Thanks!