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

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Date:   Wed, 31 Jul 2019 09:48:04 -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 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.

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