Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes

Andrey Ryabinin
2018-01-15T12:53:35Z

On 01/15/2018 03:46 PM, Michal Hocko wrote:
> On Mon 15-01-18 15:30:59, Andrey Ryabinin wrote:
>>
>>
>> On 01/12/2018 03:24 PM, Michal Hocko wrote:
>>> On Fri 12-01-18 00:59:38, Andrey Ryabinin wrote:
>>>> On 01/11/2018 07:29 PM, Michal Hocko wrote:
>>> [...]
>>>>> I do not think so. Consider that this reclaim races with other
>>>>> reclaimers. Now you are reclaiming a large chunk so you might end up
>>>>> reclaiming more than necessary. SWAP_CLUSTER_MAX would reduce the over
>>>>> reclaim to be negligible.
>>>>>
>>>>
>>>> I did consider this. And I think, I already explained that sort of race in previous email.
>>>> Whether "Task B" is really a task in cgroup or it's actually a bunch of reclaimers,
>>>> doesn't matter. That doesn't change anything.
>>>
>>> I would _really_ prefer two patches here. The first one removing the
>>> hard coded reclaim count. That thing is just dubious at best. If you
>>> _really_ think that the higher reclaim target is meaningfull then make
>>> it a separate patch. I am not conviced but I will not nack it it either.
>>> But it will make our life much easier if my over reclaim concern is
>>> right and we will need to revert it. Conceptually those two changes are
>>> independent anywa.
>>>
>>
>> Ok, fair point. But what about livelock than? Don't you think that we should
>> go back to something like in V1 patch to prevent it?
>
> I am not sure what do you mean by the livelock here.
>

Livelock is when tasks in cgroup constantly allocate reclaimable memory at high rate,
and user asked to set too low unreachable limit e.g. 'echo 4096 > memory.limit_in_bytes'.

We will loop indefinitely in mem_cgroup_resize_limit(), because try_to_free_mem_cgroup_pages() != 0
(as long as cgroup tasks generate new reclaimable pages fast enough).