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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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 <8021be4426fdafdce83517194112f43009fb9f6d.camel@surriel.com>
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 <76dbc397e21d64da75cd07d90b3ca15ca50d6fbb.camel@surriel.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <3307e8f7-4a68-95fc-a5dd-925fd3a5f8d7@redhat.com>
Date:   Wed, 31 Jul 2019 11:49:19 -0400
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On 7/31/19 11:07 AM, Rik van Riel wrote:
> On Wed, 2019-07-31 at 10:15 -0400, Waiman Long wrote:
>> 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.
> Would it be possible to work around that by scheduling
> a work item that drops the active_mm?
>
> After all, a work item runs in a kernel thread, so by
> the time the work item is run, either the kernel will
> still be running the mm you want to get rid of as
> active_mm, or it will have already gotten rid of it
> earlier.

Yes, that may work.

Thanks,
Longman