Shakeel Butt <shakeelb@xxxxxxxxxx> writes: > On Tue, Jun 15, 2021 at 5:47 AM Alexey Gladkov <legion@xxxxxxxxxx> wrote: >> > [...] >> >> I made the second version of the patch [1], but then I had a conversation >> with Eric W. Biederman offlist. He convinced me that it is a bad idea to >> change all the values in meminfo to accommodate cgroups. But we agreed >> that MemAvailable in /proc/meminfo should respect cgroups limits. This >> field was created to hide implementation details when calculating >> available memory. You can see that it is quite widely used [2]. >> So I want to try to move in that direction. >> >> [1] https://git.kernel.org/pub/scm/linux/kernel/git/legion/linux.git/log/?h=patchset/meminfo/v2.0 >> [2] https://codesearch.debian.net/search?q=MemAvailable%3A >> > > Please see following two links on the previous discussion on having > per-memcg MemAvailable stat. > > [1] https://lore.kernel.org/linux-mm/alpine.DEB.2.22.394.2006281445210.855265@xxxxxxxxxxxxxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/linux-mm/alpine.DEB.2.23.453.2007142018150.2667860@xxxxxxxxxxxxxxxxxxxxxxxxx/ > > MemAvailable itself is an imprecise metric and involving memcg makes > this metric even more weird. The difference of semantics of swap > accounting of v1 and v2 is one source of this weirdness (I have not > checked your patch if it is handling this weirdness). The lazyfree and > deferred split pages are another source. > > So, I am not sure if complicating an already imprecise metric will > make it more useful. Making a good guess at how much memory can be allocated without triggering swapping or otherwise stressing the system is something that requires understanding our mm internals. To be able to continue changing the mm or even mm policy without introducing regressions in userspace we need to export values that userspace can use. At a first approximation that seems to look like MemAvailable. MemAvailable seems to have a good definition. Roughly the amount of memory that can be allocated without triggering swapping. Updated to include not trigger memory cgroup based swapping and I sounds good. I don't know if it will work in practice but I think it is worth exploring. I do know that hiding the implementation details and providing userspace with information it can directly use seems like the programming model that needs to be explored. Most programs should not care if they are in a memory cgroup, etc. Programs, load management systems, and even balloon drivers have a legitimately interest in how much additional load can be placed on a systems memory. A version of this that I remember working fairly well is free space on compressed filesystems. As I recall compressed filesystems report the amount of uncompressed space that is available (an underestimate). This results in the amount of space consumed going up faster than the free space goes down. We can't do exactly the same thing with our memory usability estimate, but having our estimate be a reliable underestimate might be enough to avoid problems with reporting too much memory as available to userspace. I know that MemAvailable already does that /2 so maybe it is already aiming at being an underestimate. Perhaps we need some additional accounting to help create a useful metric for userspace as well. I don't know the final answer. I do know that not designing an interface that userspace can use to deal with it's legitimate concerns is sticking our collective heads in the sand and wishing the problem will go away. Eric
