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

[Waiman Long <longman@redhat.com>]

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.


>
> Cc: riel@surriel.com
> Cc: luto@kernel.org
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/core.c |   49 ++++++++++++++++++++++++++++++-------------------
>  1 file changed, 30 insertions(+), 19 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3214,12 +3214,8 @@ static __always_inline struct rq *
>  context_switch(struct rq *rq, struct task_struct *prev,
>  	       struct task_struct *next, struct rq_flags *rf)
>  {
> -	struct mm_struct *mm, *oldmm;
> -
>  	prepare_task_switch(rq, prev, next);
>  
> -	mm = next->mm;
> -	oldmm = prev->active_mm;
>  	/*
>  	 * For paravirt, this is coupled with an exit in switch_to to
>  	 * combine the page table reload and the switch backend into
> @@ -3228,22 +3224,37 @@ context_switch(struct rq *rq, struct tas
>  	arch_start_context_switch(prev);
>  
>  	/*
> -	 * If mm is non-NULL, we pass through switch_mm(). If mm is
> -	 * NULL, we will pass through mmdrop() in finish_task_switch().
> -	 * Both of these contain the full memory barrier required by
> -	 * membarrier after storing to rq->curr, before returning to
> -	 * user-space.
> +	 * kernel -> kernel   lazy + transfer active
> +	 *   user -> kernel   lazy + mmgrab() active
> +	 *
> +	 * kernel ->   user   switch + mmdrop() active
> +	 *   user ->   user   switch
>  	 */
> -	if (!mm) {
> -		next->active_mm = oldmm;
> -		mmgrab(oldmm);
> -		enter_lazy_tlb(oldmm, next);
> -	} else
> -		switch_mm_irqs_off(oldmm, mm, next);
> -
> -	if (!prev->mm) {
> -		prev->active_mm = NULL;
> -		rq->prev_mm = oldmm;
> +	if (!next->mm) {                                // to kernel
> +		enter_lazy_tlb(prev->active_mm, next);
> +
> +		next->active_mm = prev->active_mm;
> +		if (prev->mm)                           // from user
> +			mmgrab(prev->active_mm);
> +		else
> +			prev->active_mm = NULL;
> +	} else {                                        // to user
> +		/*
> +		 * sys_membarrier() requires an smp_mb() between setting
> +		 * rq->curr and returning to userspace.
> +		 *
> +		 * The below provides this either through switch_mm(), or in
> +		 * case 'prev->active_mm == next->mm' through
> +		 * finish_task_switch()'s mmdrop().
> +		 */
> +
> +		switch_mm_irqs_off(prev->active_mm, next->mm, next);
> +
> +		if (!prev->mm) {                        // from kernel
> +			/* will mmdrop() in finish_task_switch(). */
> +			rq->prev_mm = prev->active_mm;
> +			prev->active_mm = NULL;
> +		}
>  	}
>  
>  	rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);

This patch looks fine to me, I don't see any problem in its logic.

Acked-by: Waiman Long <longman@redhat.com>

The problem that I am trying to fix is in the kernel->kernel case where
the active_mm just get passed along. I would like to just bump the
active_mm off if it is dying. I will see what I can do to make it work
even with !CONFIG_MEMCG.

Cheers,
Longman