From: Richard Palethorpe <rpalethorpe@suse.de> To: "Michal Koutný" <mkoutny@suse.com> Cc: Shakeel Butt <shakeelb@google.com>, Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, LTP List <ltp@lists.linux.it>, Roman Gushchin <guro@fb.com>, Johannes Weiner <hannes@cmpxchg.org>, Andrew Morton <akpm@linux-foundation.org>, Christoph Lameter <cl@linux.com>, Michal Hocko <mhocko@kernel.org>, Tejun Heo <tj@kernel.org>, Vlastimil Babka <vbabka@suse.cz> Subject: Re: [PATCH v3] mm: memcg/slab: Stop reparented obj_cgroups from charging root Date: Thu, 22 Oct 2020 08:04:58 +0100 [thread overview] Message-ID: <87sga6vizp.fsf@suse.de> (raw) In-Reply-To: <20201020172402.GD46039@blackbook> Hello, Michal Koutný <mkoutny@suse.com> writes: > Hi. > > On Tue, Oct 20, 2020 at 06:52:08AM +0100, Richard Palethorpe <rpalethorpe@suse.de> wrote: >> I don't think that is relevant as we get the memcg from objcg->memcg >> which is set during reparenting. I suppose however, we can determine if >> the objcg was reparented by inspecting memcg->objcg. > +1 > >> If we just check use_hierarchy then objects directly charged to the >> memcg where use_hierarchy=0 will not be uncharged. However, maybe it is >> better to check if it was reparented and if use_hierarchy=0. > I think (I had to make a table) the yielded condition would be: > > if ((memcg->use_hierarchy && reparented) || (!mem_cgroup_is_root(memcg) && !reparented)) This looks correct, but I don't think we need to check for reparenting after all. We have the following unique scenarious: use_hierarchy=1, memcg=?, reparented=?: Because use_hierarchy=1 any descendants will have charged the current memcg, including root, so we can simply uncharge regardless of the memcg or objcg. use_hierarchy=0, memcg!=root, reparented=?: When use_hierarchy=0, objcgs are *only* reparented to root, so if we are not root the objcg must belong to us. use_hierarchy=0, memcg=root, reparented=?: We must have been reparented because root is not initialised with an objcg, but use_hierarchy=0 so we can not uncharge. So I believe that the following is sufficient. if (memcg->use_hierarchy || !mem_cgroup_is_root(memcg)) > __memcg_kmem_uncharge(memcg, nr_pages); > > (I admit it's not very readable.) > > > Michal For the record, I did create the following patch which checks for reparenting, but it appears to be unecessary. ---- diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 6877c765b8d0..0285f760f003 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -257,6 +257,14 @@ struct cgroup_subsys_state *vmpressure_to_css(struct vmpressure *vmpr) #ifdef CONFIG_MEMCG_KMEM extern spinlock_t css_set_lock; +/* Assumes objcg originated from a descendant of memcg or is memcg's */ +static bool obj_cgroup_did_charge(const struct obj_cgroup *objcg, + const struct mem_cgroup *memcg) +{ + return memcg->use_hierarchy || + rcu_access_pointer(memcg->objcg) == objcg; +} + static void obj_cgroup_release(struct percpu_ref *ref) { struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt); @@ -291,7 +299,7 @@ static void obj_cgroup_release(struct percpu_ref *ref) spin_lock_irqsave(&css_set_lock, flags); memcg = obj_cgroup_memcg(objcg); - if (nr_pages) + if (nr_pages && obj_cgroup_did_charge(objcg, memcg)) __memcg_kmem_uncharge(memcg, nr_pages); list_del(&objcg->list); mem_cgroup_put(memcg); @@ -3100,6 +3108,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes) static void drain_obj_stock(struct memcg_stock_pcp *stock) { struct obj_cgroup *old = stock->cached_objcg; + struct mem_cgroup *memcg; if (!old) return; @@ -3110,7 +3119,9 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock) if (nr_pages) { rcu_read_lock(); - __memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages); + memcg = obj_cgroup_memcg(old); + if (obj_cgroup_did_charge(old, memcg)) + __memcg_kmem_uncharge(memcg, nr_pages); rcu_read_unlock(); } -- Thank you, Richard.
next prev parent reply other threads:[~2020-10-22 7:05 UTC|newest] Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-10-14 19:07 [RFC PATCH] " Richard Palethorpe 2020-10-14 20:08 ` Roman Gushchin 2020-10-16 5:40 ` Richard Palethorpe 2020-10-16 9:47 ` Michal Koutný 2020-10-16 10:41 ` Richard Palethorpe 2020-10-16 15:05 ` Richard Palethorpe 2020-10-16 17:26 ` Michal Koutný 2020-10-16 14:53 ` Johannes Weiner 2020-10-16 17:02 ` Roman Gushchin 2020-10-16 17:15 ` Michal Koutný 2020-10-19 8:45 ` Richard Palethorpe 2020-10-19 9:58 ` [PATCH v3] " Richard Palethorpe 2020-10-19 16:58 ` Shakeel Butt 2020-10-20 5:52 ` Richard Palethorpe 2020-10-20 13:49 ` Richard Palethorpe 2020-10-20 16:56 ` Shakeel Butt 2020-10-21 20:32 ` Roman Gushchin 2020-10-20 17:24 ` Michal Koutný 2020-10-22 7:04 ` Richard Palethorpe [this message] 2020-10-22 12:28 ` [PATCH v4] " Richard Palethorpe 2020-10-22 16:37 ` Shakeel Butt 2020-10-22 17:25 ` Roman Gushchin 2020-10-22 23:59 ` Shakeel Butt 2020-10-23 0:40 ` Roman Gushchin 2020-10-23 15:44 ` Johannes Weiner 2020-10-23 16:41 ` Shakeel Butt 2020-10-26 7:32 ` Richard Palethorpe 2020-10-26 23:14 ` Roman Gushchin 2020-10-19 22:28 ` [RFC PATCH] " Roman Gushchin 2020-10-20 6:04 ` Richard Palethorpe 2020-10-20 12:02 ` Richard Palethorpe 2020-10-20 14:48 ` Richard Palethorpe 2020-10-20 16:27 ` Michal Koutný 2020-10-20 17:07 ` Roman Gushchin 2020-10-20 18:18 ` Johannes Weiner 2020-10-21 19:33 ` Roman Gushchin 2020-10-23 16:30 ` Johannes Weiner 2020-11-10 1:27 ` Roman Gushchin 2020-11-10 15:11 ` Shakeel Butt 2020-11-10 19:13 ` Roman Gushchin 2020-11-20 17:46 ` Michal Koutný 2020-11-03 13:22 ` Michal Hocko 2020-11-03 21:30 ` Roman Gushchin 2020-10-20 16:55 ` Shakeel Butt 2020-10-20 17:17 ` Roman Gushchin
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