On Thu, 31 Oct 2024 15:03:34 -0400 Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote: > Andrew -- I am sorry to ask again, but do you think you can replace > the 3rd section in the patch (3. Implementation Details) with the > following paragraphs? No problem. : This patch introduces a new counter to memory.stat that tracks hugeTLB : usage, only if hugeTLB accounting is done to memory.current. This feature : is enabled the same way hugeTLB accounting is enabled, via the : memory_hugetlb_accounting mount flag for cgroupsv2. : : 1. Why is this patch necessary? : Currently, memcg hugeTLB accounting is an opt-in feature [1] that adds : hugeTLB usage to memory.current. However, the metric is not reported in : memory.stat. Given that users often interpret memory.stat as a breakdown : of the value reported in memory.current, the disparity between the two : reports can be confusing. This patch solves this problem by including the : metric in memory.stat as well, but only if it is also reported in : memory.current (it would also be confusing if the value was reported in : memory.stat, but not in memory.current) : : Aside from the consistency between the two files, we also see benefits in : observability. Userspace might be interested in the hugeTLB footprint of : cgroups for many reasons. For instance, system admins might want to : verify that hugeTLB usage is distributed as expected across tasks: i.e. : memory-intensive tasks are using more hugeTLB pages than tasks that don't : consume a lot of memory, or are seen to fault frequently. Note that this : is separate from wanting to inspect the distribution for limiting purposes : (in which case, hugeTLB controller makes more sense). : : 2. We already have a hugeTLB controller. Why not use that? It is true : that hugeTLB tracks the exact value that we want. In fact, by enabling : the hugeTLB controller, we get all of the observability benefits that I : mentioned above, and users can check the total hugeTLB usage, verify if it : is distributed as expected, etc. : : 3. Implementation Details: : In the alloc / free hugetlb functions, we call lruvec_stat_mod_folio : regardless of whether memcg accounts hugetlb. mem_cgroup_commit_charge : which is called from alloc_hugetlb_folio will set memcg for the folio : only if the CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING cgroup mount option is : used, so lruvec_stat_mod_folio accounts per-memcg hugetlb counters only : if the feature is enabled. Regardless of whether memcg accounts for : hugetlb, the newly added global counter is updated and shown in : /proc/vmstat. : : The global counter is added because vmstats is the preferred framework : for cgroup stats. It makes stat items consistent between global and : cgroups. It also provides a per-node breakdown, which is useful. : Because it does not use cgroup-specific hooks, we also keep generic MM : code separate from memcg code. : : With this said, there are 2 problems: : (a) They are still not reported in memory.stat, which means the : disparity between the memcg reports are still there. : (b) We cannot reasonably expect users to enable the hugeTLB controller : just for the sake of hugeTLB usage reporting, especially since : they don't have any use for hugeTLB usage enforcing [2]. : : [1] https://lore.kernel.org/all/20231006184629.155543-1-nphamcs@xxxxxxxxx/ : [2] Of course, we can't make a new patch for every feature that can be : duplicated. However, since the existing solution of enabling the : hugeTLB controller is an imperfect solution that still leaves a : discrepancy between memory.stat and memory.curent, I think that it : is reasonable to isolate the feature in this case.
