On Thu, 30 Nov 2017, Andrew Morton wrote: > > This patchset makes the OOM killer cgroup-aware. > > Thanks, I'll grab these. > > There has been controversy over this patchset, to say the least. I > can't say that I followed it closely! Could those who still have > reservations please summarise their concerns and hopefully suggest a > way forward? > Yes, I'll summarize what my concerns have been in the past and what they are wrt the patchset as it stands in -mm. None of them originate from my current usecase or anticipated future usecase of the oom killer for system-wide or memcg-constrained oom conditions. They are based purely on the patchset's use of an incomplete and unfair heuristic for deciding which cgroup to target. I'll also suggest simple changes to the patchset, which I have in the past, that can be made to address all of these concerns. 1. The unfair comparison of the root mem cgroup vs leaf mem cgroups The patchset uses two different heuristics to compare root and leaf mem cgroups and scores them based on number of pages. For the root mem cgroup, it totals the /proc/pid/oom_score of all processes attached: that's based on rss, swap, pgtables, and, most importantly, oom_score_adj. For leaf mem cgroups, it's based on that memcg's anonymous, unevictable, unreclaimable slab, kernel stack, and swap counters. These can be wildly different independent of /proc/pid/oom_score_adj, but the most obvious unfairness comes from users who tune oom_score_adj. An example: start a process that faults 1GB of anonymous memory and leave it attached to the root mem cgroup. Start six more processes that each fault 1GB of anonymous memory and attached them to a leaf mem cgroup. Set all processes to have /proc/pid/oom_score_adj of 1000. System oom kill will always kill the 1GB process attached to the root mem cgroup. It's because oom_badness() relies on /proc/pid/oom_score_adj, which is used to evaluate the root mem cgroup, and leaf mem cgroups completely disregard it. In this example, the leaf mem cgroup's score is 1,573,044, the number of pages for the 6GB of faulted memory. The root mem cgroup's score is 12,652,907, eight times larger even though its usage is six times smaller. This is caused by the patchset disregarding oom_score_adj entirely for leaf mem cgroups and relying on it heavily for the root mem cgroup. It's the complete opposite result of what the cgroup aware oom killer advertises. It also works the other way, if a large memory hog is attached to the root mem cgroup but has a negative oom_score_adj it is never killed and random processes are nuked solely because they happened to be attached to a leaf mem cgroup. This behavior wrt oom_score_adj is completely undocumented, so I can't presume that it is either known nor tested. Solution: compare the root mem cgroup and leaf mem cgroups equally with the same criteria by doing hierarchical accounting of usage and subtracting from total system usage to find root usage. 2. Evading the oom killer by attaching processes to child cgroups Any cgroup on the system can attach all their processes to individual child cgroups. This is functionally the same as doing for i in $(cat cgroup.procs); do mkdir $i; echo $i > $i/cgroup.procs; done without the no internal process constraint introduced with cgroup v2. All child cgroups are evaluated based on their own usage: all anon, unevictable, and unreclaimable slab as described previously. It requires an individual cgroup to be the single largest consumer to be targeted by the oom killer. An example: allow users to manage two different mem cgroup hierarchies limited to 100GB each. User A uses 10GB of memory and user B uses 90GB of memory in their respective hierarchies. On a system oom condition, we'd expect at least one process from user B's hierarchy would always be oom killed with the cgroup aware oom killer. In fact, the changelog explicitly states it solves an issue where "1) There is no fairness between containers. A small container with few large processes will be chosen over a large one with huge number of small processes." The opposite becomes true, however, if user B creates child cgroups and distributes its processes such that each child cgroup's usage never exceeds 10GB of memory. This can either be done intentionally to purposefully have a low cgroup memory footprint to evade the oom killer or unintentionally with cgroup v2 to allow those individual processes to be constrained by other cgroups in a single hierarchy model. User A, using 10% of his memory limit, is always oom killed instead of user B, using 90% of his memory limit. Others have commented its still possible to do this with a per-process model if users split their processes into many subprocesses with small memory footprints. Solution: comparing cgroups must be done hierarchically. Neither user A nor user B can evade the oom killer because targeting is done based on the total hierarchical usage rather than individual cgroups in their hierarchies. 3. Userspace has zero control over oom kill selection in leaf mem cgroups Unlike using /proc/pid/oom_score_adj to bias or prefer certain processes from the oom killer, the cgroup aware oom killer does not provide any solution for the user to protect leaf mem cgroups. This is a result of leaf mem cgroups being evaluated based on their anon, unevictable, and unreclaimable slab usage and disregarding any user tunable. Absent the cgroup aware oom killer, users have the ability to strongly prefer a process is oom killed (/proc/pid/oom_score_adj = 1000) or strongly bias against a process (/proc/pid/oom_score_adj = -999). An example: a process knows its going to use a lot of memory, so it sets /proc/self/oom_score_adj to 1000. It wants to be killed first to avoid distrupting any other process. If it's attached to the root mem cgroup, it will be oom killed. If it's attached to a leaf mem cgroup by an admin outside its control, it will never be oom killed unless that cgroup's usage is the largest single cgroup usage on the system. The reverse also is true for processes that the admin does not want to be oom killed: set /proc/pid/oom_score_adj to -999, but it will *always* be oom killed if its cgroup has the highest usage on the system. The result is that both admins and users have lost all control over which processes are oom killed. They are left with only one alternative: set /proc/pid/oom_score_adj to -1000 to completely disable a process from oom kill. It doesn't address the issue at all for memcg-constrained oom conditions since no processes are killable anymore, and risks panicking the system if it is the only process left on the system. A process preferring that it is first in line for oom kill simply cannot volunteer anymore. Solution: allow users and admins to control oom kill selection by introducing a memory.oom_score_adj to affect the oom score of that mem cgroup, exactly the same as /proc/pid/oom_score_adj affects the oom score of a process. I proposed a solution in https://marc.info/?l=linux-kernel&m=150956897302725, which was never responded to, for all of these issues. The idea is to do hierarchical accounting of mem cgroup hierarchies so that the hierarchy is traversed comparing total usage at each level to select target cgroups. Admins and users can use memory.oom_score_adj to influence that decisionmaking at each level. This solves #1 because mem cgroups can be compared based on the same classes of memory and the root mem cgroup's usage can be fairly compared by subtracting top-level mem cgroup usage from system usage. All of the criteria used to evaluate a leaf mem cgroup has a reasonable system-wide counterpart that can be used to do the simple subtraction. This solves #2 because evaluation is done hierarchically so that distributing processes over a set of child cgroups either intentionally or unintentionally no longer evades the oom killer. Total usage is always accounted to the parent and there is no escaping this criteria for users. This solves #3 because it allows admins to protect important processes in cgroups that are supposed to use, for example, 75% of system memory without it unconditionally being selected for oom kill but still oom kill if it exceeds a certain threshold. In this sense, the cgroup aware oom killer, as currently implemented, is selling mem cgroups short by requiring the user to accept that the important process will be oom killed iff it uses mem cgroups and isn't attached to root. It also allows users to actually volunteer to be oom killed first without majority usage. It has come up time and time again that this support can be introduced on top of the cgroup oom killer as implemented. It simply cannot. For admins and users to have control over decisionmaking, it needs a oom_score_adj type tunable that cannot change semantics from kernel version to kernel version and without polluting the mem cgroup filesystem. That, in my suggestion, is an adjustment on the amount of total hierarchical usage of each mem cgroup at each level of the hierarchy. That requires that the heuristic uses hierarchical usage rather than considering each cgroup as independent consumers as it does today. We need to implement that heuristic and introduce userspace influence over oom kill selection now rather than later because its implementation changes how this patchset is implemented. I can implement these changes, if preferred, on top of the current patchset, but I do not believe we want inconsistencies between kernel versions that introduce user visible changes for the sole reason that this current implementation is incomplete and unfair. We can implement and introduce it once without behavior changing later because the core heuristic has necessarily changed. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
