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    SubjectRe: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes
    On Mon 15-01-18 15:53:35, Andrey Ryabinin wrote:
    >
    >
    > 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'.

    OK, I wasn't sure. The reclaim target, however, doesn't have a direct
    influence on this, though.

    > 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).

    I do not thing this is a real problem. The context is interruptible and
    I would even consider it safer to keep retrying than simply failing
    prematurely. My experience tells me that basically any hard coded retry
    loop in the kernel is wrong.

    --
    Michal Hocko
    SUSE Labs

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