Re: [PATCH 1/2] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes

[Shakeel Butt <shakeelb@google.com>]

On Wed, Dec 20, 2017 at 3:34 AM, Michal Hocko <mhocko@kernel.org> 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.

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