Re: [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads

[Rik van Riel <riel@surriel.com>]

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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.

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