Hello Michal, On Thu, Feb 04, 2021 at 03:19:17PM +0100, Michal Hocko wrote: > On Tue 02-02-21 13:47:45, Johannes Weiner wrote: > > Replace the memory controller's custom hierarchical stats code with > > the generic rstat infrastructure provided by the cgroup core. > > > > The current implementation does batched upward propagation from the > > write side (i.e. as stats change). The per-cpu batches introduce an > > error, which is multiplied by the number of subgroups in a tree. In > > systems with many CPUs and sizable cgroup trees, the error can be > > large enough to confuse users (e.g. 32 batch pages * 32 CPUs * 32 > > subgroups results in an error of up to 128M per stat item). This can > > entirely swallow allocation bursts inside a workload that the user is > > expecting to see reflected in the statistics. > > > > In the past, we've done read-side aggregation, where a memory.stat > > read would have to walk the entire subtree and add up per-cpu > > counts. This became problematic with lazily-freed cgroups: we could > > have large subtrees where most cgroups were entirely idle. Hence the > > switch to change-driven upward propagation. Unfortunately, it needed > > to trade accuracy for speed due to the write side being so hot. > > > > Rstat combines the best of both worlds: from the write side, it > > cheaply maintains a queue of cgroups that have pending changes, so > > that the read side can do selective tree aggregation. This way the > > reported stats will always be precise and recent as can be, while the > > aggregation can skip over potentially large numbers of idle cgroups. > > > > This adds a second vmstats to struct mem_cgroup (MEMCG_NR_STAT + > > NR_VM_EVENT_ITEMS) to track pending subtree deltas during upward > > aggregation. It removes 3 words from the per-cpu data. It eliminates > > memcg_exact_page_state(), since memcg_page_state() is now exact. > > I am still digesting details and need to look deeper into how rstat > works but removing our own stats is definitely a good plan. Especially > when there are existing limitations and problems that would need fixing. > > Just to check that my high level understanding is correct. The > transition is effectivelly removing a need to manually sync counters up > the hierarchy and partially outsources that decision to rstat core. The > controller is responsible just to tell the core how that syncing is done > (e.g. which specific counters etc). Yes, exactly. rstat implements a tree of cgroups that have local changes pending, and a flush walk on that tree. But it's all driven by the controller. memcg needs to tell rstat 1) when stats in a local cgroup change e.g. when we do mod_memcg_state() (cgroup_rstat_updated), 2) when to flush, e.g. before a memory.stat read (cgroup_rstat_flush), and 3) how to flush one cgroup's per-cpu state and propagate it upward to the parent during rstat's flush walk (.css_rstat_flush). > Excplicit flushes are needed when you want an exact value (e.g. when > values are presented to the userspace). I do not see any flushes to > be done by the core pro-actively except for clean up on a release. > > Is the above correct understanding? Yes, that's correct.
