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Subject: Re: [PATCH v3] sched/core: Don't use dying mm as active_mm of
 kthreads
To:     Rik van Riel <riel@surriel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Phil Auld <pauld@redhat.com>, Michal Hocko <mhocko@kernel.org>
References: <20190729210728.21634-1-longman@redhat.com>
 <ec9effc07a94b28ecf364de40dee183bcfb146fc.camel@surriel.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <3e2ff4c9-c51f-8512-5051-5841131f4acb@redhat.com>
Date:   Mon, 29 Jul 2019 17:42:20 -0400
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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