Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of kthreads

[Waiman Long <longman@redhat.com>]

On 7/29/19 10:27 AM, Peter Zijlstra wrote:
> On Mon, Jul 29, 2019 at 10:52:35AM +0200, Peter Zijlstra wrote:
>> On Sat, Jul 27, 2019 at 01:10:47PM -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 memory and other resources like swap
>>> space that cannot be freed.
>> Sure, but this has been so 'forever', why is it a problem now?
>>
>>> Fix that by forcing the kernel threads to use init_mm as the active_mm
>>> if the previous active_mm is dying.
>>>
>>> The determination of a dying mm is based on the absence of an owning
>>> task. The selection of the owning task only happens with the CONFIG_MEMCG
>>> option. Without that, there is no simple way to determine the life span
>>> of a given mm. So it falls back to the old behavior.
>>>
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>> ---
>>>  include/linux/mm_types.h | 15 +++++++++++++++
>>>  kernel/sched/core.c      | 13 +++++++++++--
>>>  mm/init-mm.c             |  4 ++++
>>>  3 files changed, 30 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>>> index 3a37a89eb7a7..32712e78763c 100644
>>> --- a/include/linux/mm_types.h
>>> +++ b/include/linux/mm_types.h
>>> @@ -623,6 +623,21 @@ static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
>>>  	return atomic_read(&mm->tlb_flush_pending) > 1;
>>>  }
>>>  
>>> +#ifdef CONFIG_MEMCG
>>> +/*
>>> + * A mm is considered dying if there is no owning task.
>>> + */
>>> +static inline bool mm_dying(struct mm_struct *mm)
>>> +{
>>> +	return !mm->owner;
>>> +}
>>> +#else
>>> +static inline bool mm_dying(struct mm_struct *mm)
>>> +{
>>> +	return false;
>>> +}
>>> +#endif
>>> +
>>>  struct vm_fault;
>> Yuck. So people without memcg will still suffer the terrible 'whatever
>> it is this patch fixes'.
> Also; why then not key off that owner tracking to free the resources
> (and leave the struct mm around) and avoid touching this scheduling
> hot-path ?

The resources are pinned by the reference count. Making a special case
will certainly mess up the existing code.

It is actually a problem for systems that are mostly idle. Only the
kernel->kernel case needs to be updated. If the CPUs isn't busy running
user tasks, a little bit more overhead shouldn't really hurt IMHO.

Cheers,
Longman