Re: [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads

[Waiman Long <longman@redhat.com>]

On 7/30/19 3:24 AM, Michal Hocko wrote:
> On Mon 29-07-19 17:42:20, Waiman Long wrote:
>> On 7/29/19 5:21 PM, Rik van Riel wrote:
>>> On Mon, 2019-07-29 at 17:07 -0400, Waiman Long wrote:
>>>> It was found that a dying mm_struct where the owning task has exited
>>>> can stay on as active_mm of kernel threads as long as no other user
>>>> tasks run on those CPUs that use it as active_mm. This prolongs the
>>>> life time of dying mm holding up some resources that cannot be freed
>>>> on a mostly idle system.
>>> On what kernels does this happen?
>>>
>>> Don't we explicitly flush all lazy TLB CPUs at exit
>>> time, when we are about to free page tables?
>> There are still a couple of calls that will be done until mm_count
>> reaches 0:
>>
>> - mm_free_pgd(mm);
>> - destroy_context(mm);
>> - mmu_notifier_mm_destroy(mm);
>> - check_mm(mm);
>> - put_user_ns(mm->user_ns);
>>
>> These are not big items, but holding it off for a long time is still not
>> a good thing.
> It would be helpful to give a ball park estimation of how much that
> actually is. If we are talking about few pages worth of pages per idle
> cpu in the worst case then I am not sure we want to find an elaborate
> way around that. We are quite likely having more in per-cpu caches in
> different subsystems already. It is also quite likely that large
> machines with many CPUs will have a lot of memory as well.

I think they are relatively small. So I am not going to pursue it
further at this point.

Cheers,
Longman