From: Roman Gushchin <email@example.com> To: Johannes Weiner <firstname.lastname@example.org> Cc: "Michal Koutný" <email@example.com>, "Richard Palethorpe" <firstname.lastname@example.org>, email@example.com, "Andrew Morton" <firstname.lastname@example.org>, "Shakeel Butt" <email@example.com>, "Christoph Lameter" <firstname.lastname@example.org>, "Michal Hocko" <email@example.com>, "Tejun Heo" <firstname.lastname@example.org>, "Vlastimil Babka" <email@example.com>, firstname.lastname@example.org, email@example.com Subject: Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from charging root Date: Fri, 16 Oct 2020 10:02:45 -0700 [thread overview] Message-ID: <20201016170245.GB230727@carbon.DHCP.thefacebook.com> (raw) In-Reply-To: <20201016145308.GA312010@cmpxchg.org> On Fri, Oct 16, 2020 at 10:53:08AM -0400, Johannes Weiner wrote: > On Fri, Oct 16, 2020 at 11:47:02AM +0200, Michal Koutný wrote: > > Hello. > > > > On Wed, Oct 14, 2020 at 08:07:49PM +0100, Richard Palethorpe <firstname.lastname@example.org> wrote: > > > SLAB objects which outlive their memcg are moved to their parent > > > memcg where they may be uncharged. However if they are moved to the > > > root memcg, uncharging will result in negative page counter values as > > > root has no page counters. > > Why do you think those are reparented objects? If those are originally > > charged in a non-root cgroup, then the charge value should be propagated up the > > hierarchy, including root memcg, so if they're later uncharged in root > > after reparenting, it should still break even. (Or did I miss some stock > > imbalance?) > > Looking a bit closer at this code, it's kind of a mess right now. > > The central try_charge() function charges recursively all the way up > to and including the root. But not if it's called directly on the > root, in which case it bails and does nothing. > > kmem and objcg use try_charge(), so they have the same > behavior. get_obj_cgroup_from_current() does it's own redundant > filtering for root_mem_cgroup, whereas get_mem_cgroup_from_current() > does not, but its callsite __memcg_kmem_charge_page() does. > > We should clean this up one way or another: either charge the root or > don't, but do it consistently. +1 > > Since we export memory.stat at the root now, we should probably just > always charge the root instead of special-casing it all over the place > and risking bugs. Hm, we export memory.stat but not memory.current. Charging the root memcg seems to be an extra atomic operation, which can be avoided. I wonder if we can handle it in page_counter.c, so there will be a single place where we do the check. > > Indeed, it looks like there is at least one bug where the root-level > memory.stat shows non-root slab objects, but not root ones, whereas it > shows all anon and cache pages, root or no root. I'll take a look.
next prev parent reply other threads:[~2020-10-16 17:03 UTC|newest] Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-10-14 19:07 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 [this message] 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 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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