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Mon, 3 Feb 2020 21:38:53 +0000 Date: Mon, 3 Feb 2020 13:38:47 -0800 From: Roman Gushchin To: Johannes Weiner CC: Michal Hocko , Andrew Morton , Tejun Heo , , , , Subject: Re: [PATCH v2 1/3] mm: memcontrol: fix memory.low proportional distribution Message-ID: <20200203213847.GC6781@xps.dhcp.thefacebook.com> References: <20191219200718.15696-1-hannes@cmpxchg.org> <20191219200718.15696-2-hannes@cmpxchg.org> <20200130114929.GT24244@dhcp22.suse.cz> <20200203212136.GC6380@cmpxchg.org> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200203212136.GC6380@cmpxchg.org> X-ClientProxiedBy: MN2PR05CA0038.namprd05.prod.outlook.com (2603:10b6:208:236::7) To BYAPR15MB2631.namprd15.prod.outlook.com (2603:10b6:a03:150::19) MIME-Version: 1.0 Received: from xps.dhcp.thefacebook.com (2620:10d:c091:480::a012) by MN2PR05CA0038.namprd05.prod.outlook.com (2603:10b6:208:236::7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2707.11 via Frontend Transport; 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I have got back to email threads when > > the effective hierarchical protection has been proposed. > > http://lkml.kernel.org/r/20180320223353.5673-1-guro@fb.com talks about > > some "leaf memory cgroups are more valuable than others" which sounds ok > > but it doesn't explain why children have to overcommit parent in the > > first place. Do we have any other documentation to explain the usecase? > > I don't think we properly documented it, no. Maybe Roman can elaborate > on that a bit more since he added it. There is simple no way to prevent it. Cgroup v2 UI allows to set any values, and they are never changed automatically on e.g. parent's value change. So the system has to perform reasonably for all possible configurations. Before introducing of effective protections it was way too defensive: it was super easy to disable the protection by exceeding the protection value on some level. So it wasn't possible to rely on it in production. So disabling the protection in case of over-commitment isn't a good option too. Thanks!