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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>
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 <c91e6104acaef118ae09e4b4b0c70232c4583293.camel@surriel.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <01125822-c883-18ce-42e4-942a4f28c128@redhat.com>
Date:   Wed, 31 Jul 2019 10:15:17 -0400
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On 7/31/19 9:48 AM, Rik van Riel wrote:
> On Tue, 2019-07-30 at 17:01 -0400, Waiman Long wrote:
>> On 7/29/19 8:26 PM, Rik van Riel wrote:
>>> On Mon, 2019-07-29 at 17:42 -0400, Waiman Long wrote:
>>>
>>>> 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.
>>> OK. On what kernel are you seeing this?
>>>
>>> On current upstream, the code in native_flush_tlb_others()
>>> will send a TLB flush to every CPU in mm_cpumask() if page
>>> table pages have been freed.
>>>
>>> That should cause the lazy TLB CPUs to switch to init_mm
>>> when the exit->zap_page_range path gets to the point where
>>> it frees page tables.
>>>
>> I was using the latest upstream 5.3-rc2 kernel. It may be the case
>> that
>> the mm has been switched, but the mm_count field of the active_mm of
>> the
>> kthread is not being decremented until a user task runs on a CPU.
> Is that something we could fix from the TLB flushing
> code?
>
> When switching to init_mm, drop the refcount on the
> lazy mm?
>
> That way that overhead is not added to the context
> switching code.

I have thought about that. That will require changing the active_mm of
the current task to point to init_mm, for example. Since TLB flush is
done in interrupt context, proper coordination between interrupt and
process context will require some atomic instruction which will defect
the purpose.

Cheers,
Longman