On 12/20/2017 09:15 PM, Shakeel Butt wrote: > On Wed, Dec 20, 2017 at 3:34 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: >> On Wed 20-12-17 14:32:19, Andrey Ryabinin wrote: >>> On 12/20/2017 01:33 PM, Michal Hocko wrote: >>>> On Wed 20-12-17 13:24:28, Andrey Ryabinin wrote: >>>>> mem_cgroup_resize_[memsw]_limit() tries to free only 32 (SWAP_CLUSTER_MAX) >>>>> pages on each iteration. This makes practically impossible to decrease >>>>> limit of memory cgroup. Tasks could easily allocate back 32 pages, >>>>> so we can't reduce memory usage, and once retry_count reaches zero we return >>>>> -EBUSY. >>>>> >>>>> It's easy to reproduce the problem by running the following commands: >>>>> >>>>> mkdir /sys/fs/cgroup/memory/test >>>>> echo $$ >> /sys/fs/cgroup/memory/test/tasks >>>>> cat big_file > /dev/null & >>>>> sleep 1 && echo $((100*1024*1024)) > /sys/fs/cgroup/memory/test/memory.limit_in_bytes >>>>> -bash: echo: write error: Device or resource busy >>>>> >>>>> Instead of trying to free small amount of pages, it's much more >>>>> reasonable to free 'usage - limit' pages. >>>> >>>> But that only makes the issue less probable. It doesn't fix it because >>>> if (curusage >= oldusage) >>>> retry_count--; >>>> can still be true because allocator might be faster than the reclaimer. >>>> Wouldn't it be more reasonable to simply remove the retry count and keep >>>> trying until interrupted or we manage to update the limit. >>> >>> But does it makes sense to continue reclaiming even if reclaimer can't >>> make any progress? I'd say no. "Allocator is faster than reclaimer" >>> may be not the only reason for failed reclaim. E.g. we could try to >>> set limit lower than amount of mlock()ed memory in cgroup, retrying >>> reclaim would be just a waste of machine's resources. Or we simply >>> don't have any swap, and anon > new_limit. Should be burn the cpu in >>> that case? >> >> We can check the number of reclaimed pages and go EBUSY if it is 0. >> >>>> Another option would be to commit the new limit and allow temporal overcommit >>>> of the hard limit. New allocations and the limit update paths would >>>> reclaim to the hard limit. >>>> >>> >>> It sounds a bit fragile and tricky to me. I wouldn't go that way >>> without unless we have a very good reason for this. >> >> I haven't explored this, to be honest, so there may be dragons that way. >> I've just mentioned that option for completness. >> > > We already do this for cgroup-v2's memory.max. So, I don't think it is > fragile or tricky. > It has a potential to break userspace expectation. Userspace might expect that lowering limit_in_bytes too much fails with EBUSY and doesn't trigger OOM killer. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
