On Wed, Oct 23, 2024 at 02:17:27PM -0700, Yosry Ahmed wrote: > On Wed, Oct 23, 2024 at 2:15 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > Hi Joshua, > > > > On Wed, Oct 23, 2024 at 1:34 PM Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote: > > > > > > Changelog > > > v2: > > > * Enables the feature only if memcg accounts for hugeTLB usage > > > * Moves the counter from memcg_stat_item to node_stat_item > > > * Expands on motivation & justification in commitlog > > > * Added Suggested-by: Nhat Pham > > > > Changelogs usually go at the end, after ---, not as part of the commit > > log itself. > > > > > > > > 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 consistentcy between the two files, we also see benefits > > > > consistency* > > > > > 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 is seen to fault frequently. Note that > > > > are* seen > > > > > 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. > > > > > > 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 exsting solution of enabling the > > > > existing* > > > > > 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. > > > > > > Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx> > > > Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx> > > > > You should also CC linux-mm on such patches. > > +roman.gushchin@xxxxxxxxx > > CCing Roman's correct email. Funny enough I suggested a similar functionality back to 2017: https://lkml.iu.edu/hypermail/linux/kernel/1711.1/04891.html , but it was rejected at that time. So yeah, it still sounds like a good idea to me. As I understand, there is a new version of this patchset pending, I'll wait for it for the review. Thank you for looping me in!
