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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>
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 <8021be4426fdafdce83517194112f43009fb9f6d.camel@surriel.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <b5a462b8-8ef6-6d2c-89aa-b5009c194000@redhat.com>
Date:   Tue, 30 Jul 2019 17:01:38 -0400
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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.


>>> 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.
> mm switching is also done in flush_tlb_func_common,
> if the CPU received a TLB shootdown IPI while in lazy
> TLB mode.
>
I see.

Cheers,
Longman