On Wed, Apr 17, 2019 at 8:07 PM Roman Gushchin <guro@xxxxxx> 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 <guro@xxxxxx> 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 <guroan@xxxxxxxxx> 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_caches > > > > > 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 too. > > > > > > > > > > > > > > > > > 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 you > > > > 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_online. > > > > > > 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. > The reason I knew about that race is because I tried something similar but for different use-case: https://lkml.org/lkml/2018/3/26/472 > 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 size > to the kmem_cache object. Still an option, of course. > I kind of prefer this option. > 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? Should it be just reclaim or anything that can reschedule the current thread? 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. 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. Shakeel
