Re: [PATCH] sched: Clean up active_mm reference counting

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 29, 2019 at 11:16:55AM -0400, Waiman Long wrote:
> On 7/29/19 10:24 AM, Peter Zijlstra wrote:
> > On Mon, Jul 29, 2019 at 10:52:35AM +0200, Peter Zijlstra wrote:
> >
> > ---
> > Subject: sched: Clean up active_mm reference counting
> > From: Peter Zijlstra <peterz@infradead.org>
> > Date: Mon Jul 29 16:05:15 CEST 2019
> >
> > The current active_mm reference counting is confusing and sub-optimal.
> >
> > Rewrite the code to explicitly consider the 4 separate cases:
> >
> >     user -> user
> >
> > 	When switching between two user tasks, all we need to consider
> > 	is switch_mm().
> >
> >     user -> kernel
> >
> > 	When switching from a user task to a kernel task (which
> > 	doesn't have an associated mm) we retain the last mm in our
> > 	active_mm. Increment a reference count on active_mm.
> >
> >   kernel -> kernel
> >
> > 	When switching between kernel threads, all we need to do is
> > 	pass along the active_mm reference.
> >
> >   kernel -> user
> >
> > 	When switching between a kernel and user task, we must switch
> > 	from the last active_mm to the next mm, hoping of course that
> > 	these are the same. Decrement a reference on the active_mm.
> >
> > The code keeps a different order, because as you'll note, both 'to
> > user' cases require switch_mm().
> >
> > And where the old code would increment/decrement for the 'kernel ->
> > kernel' case, the new code observes this is a neutral operation and
> > avoids touching the reference count.
> 
> I am aware of that behavior which is indeed redundant, but it is not
> what I am trying to fix and so I kind of leave it alone in my patch.

Oh sure; and it's not all that important either. It is jst that every
time I look at that code I get confused.

On top of that, the new is easier to rip the active_mm stuff out of,
which is where it came from.