linux-kernel - Re: [PATCH v3] sched/core: Don't use dying mm as active

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Date:   Mon, 29 Jul 2019 20:26:41 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Phil Auld <pauld@...hat.com>, Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v3] sched/core: Don't use dying mm as active_mm of
 kthreads

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.

> > 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.

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