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[213.151.95.130]) by smtp.gmail.com with ESMTPSA id d14sm8428287wru.9.2020.01.30.09.00.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jan 2020 09:00:21 -0800 (PST) Date: Thu, 30 Jan 2020 18:00:20 +0100 From: Michal Hocko To: Johannes Weiner Cc: Andrew Morton , Roman Gushchin , Tejun Heo , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH v2 3/3] mm: memcontrol: recursive memory.low protection Message-ID: <20200130170020.GZ24244@dhcp22.suse.cz> References: <20191219200718.15696-1-hannes@cmpxchg.org> <20191219200718.15696-4-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191219200718.15696-4-hannes@cmpxchg.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 19-12-19 15:07:18, Johannes Weiner wrote: > Right now, the effective protection of any given cgroup is capped by > its own explicit memory.low setting, regardless of what the parent > says. The reasons for this are mostly historical and ease of > implementation: to make delegation of memory.low safe, effective > protection is the min() of all memory.low up the tree. > > Unfortunately, this limitation makes it impossible to protect an > entire subtree from another without forcing the user to make explicit > protection allocations all the way to the leaf cgroups - something > that is highly undesirable in real life scenarios. > > Consider memory in a data center host. At the cgroup top level, we > have a distinction between system management software and the actual > workload the system is executing. Both branches are further subdivided > into individual services, job components etc. > > We want to protect the workload as a whole from the system management > software, but that doesn't mean we want to protect and prioritize > individual workload wrt each other. Their memory demand can vary over > time, and we'd want the VM to simply cache the hottest data within the > workload subtree. Yet, the current memory.low limitations force us to > allocate a fixed amount of protection to each workload component in > order to get protection from system management software in > general. This results in very inefficient resource distribution. I do agree that configuring the reclaim protection is not an easy task. Especially in a deeper reclaim hierarchy. systemd tends to create a deep and commonly shared subtrees. So having a protected workload really requires to be put directly into a new first level cgroup in practice AFAICT. That is a simpler example though. Just imagine you want to protect a certain user slice. You seem to be facing a different problem though IIUC. You know how much memory you want to protect and you do not have to care about the cgroup hierarchy up but you do not know/care how to distribute that protection among workloads running under that protection. I agree that this is a reasonable usecase. Those both problems however show that we have a more general configurability problem for both leaf and intermediate nodes. They are both a result of strong requirements imposed by delegation as you have noted above. I am thinking didn't we just go too rigid here? Delegation points are certainly a security boundary and they should be treated like that but do we really need a strong containment when the reclaim protection is under admin full control? Does the admin really have to reconfigure a large part of the hierarchy to protect a particular subtree? I do not have a great answer on how to implement this unfortunately. The best I could come up with was to add a "$inherited_protection" magic value to distinguish from an explicit >=0 protection. What's the difference? $inherited_protection would be a default and it would always refer to the closest explicit protection up the hierarchy (with 0 as a default if there is none defined). A / \ B C (low=10G) / \ D E (low = 5G) A, B don't get any protection (low=0). C gets protection (10G) and distributes the pressure to D, E when in excess. D inherits (low=10G) and E overrides the protection to 5G. That would help both usecases AFAICS while the delegation should be still possible (configure the delegation point with an explicit value). I have very likely not thought that through completely. Does that sound like a completely insane idea? Or do you think that the two usecases are simply impossible to handle at the same time? [...] -- Michal Hocko SUSE Labs