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. -- Michal Hocko SUSE Labs -- 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
