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

[Waiman Long <longman@redhat.com>]

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.

> Does this happen only on the CPU where the task in
> question is exiting, or also on other CPUs?

What I have found is that a long running process on a mostly idle system
with many CPUs is likely to cycle through a lot of the CPUs during its
lifetime and leave behind its mm in the active_mm of those CPUs.  My
2-socket test system have 96 logical CPUs. After running the test
program for a minute or so, it leaves behind its mm in about half of the
CPUs with a mm_count of 45 after exit. So the dying mm will stay until
all those 45 CPUs get new user tasks to run.


>
> If it is only on the CPU where the task is exiting,
> would the TASK_DEAD handling in finish_task_switch()
> be a better place to handle this?

I need to switch the mm off the dying one. mm switching is only done in
context_switch(). I don't think finish_task_switch() is the right place.

-Longman