Subject: Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

From: Peter Zijlstra <[email-masked]>
Date: Wed, 2 Dec 2020 13:45:19 +0100

On Wed, Dec 02, 2020 at 12:17:31PM +0100, Peter Zijlstra wrote:

> So the obvious 'improvement' here would be something like:
> 
> 	for_each_online_cpu(cpu) {
> 		p = rcu_dereference(cpu_rq(cpu)->curr;
> 		if (p->active_mm != mm)
> 			continue;
> 		__cpumask_set_cpu(cpu, tmpmask);
> 	}
> 	on_each_cpu_mask(tmpmask, ...);
> 
> The remote CPU will never switch _to_ @mm, on account of it being quite
> dead, but it is quite prone to false negatives.
> 
> Consider that __schedule() sets rq->curr *before* context_switch(), this
> means we'll see next->active_mm, even though prev->active_mm might still
> be our @mm.
> 
> Now, because we'll be removing the atomic ops from context_switch()'s
> active_mm swizzling, I think we can change this to something like the
> below. The hope being that the cost of the new barrier can be offset by
> the loss of the atomics.
> 
> Hmm ?
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 41404afb7f4c..2597c5c0ccb0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4509,7 +4509,6 @@ context_switch(struct rq *rq, struct task_struct *prev,
>  	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
> @@ -4524,6 +4523,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
>  		 * case 'prev->active_mm == next->mm' through
>  		 * finish_task_switch()'s mmdrop().
>  		 */
> +		next->active_mm = next->mm;
>  		switch_mm_irqs_off(prev->active_mm, next->mm, next);

I think that next->active_mm store should be after switch_mm(),
otherwise we still race.

>  
>  		if (!prev->mm) {                        // from kernel
> @@ -5713,11 +5713,9 @@ static void __sched notrace __schedule(bool preempt)
>  
>  	if (likely(prev != next)) {
>  		rq->nr_switches++;
> -		/*
> -		 * RCU users of rcu_dereference(rq->curr) may not see
> -		 * changes to task_struct made by pick_next_task().
> -		 */
> -		RCU_INIT_POINTER(rq->curr, next);
> +
> +		next->active_mm = prev->active_mm;
> +		rcu_assign_pointer(rq->curr, next);
>  		/*
>  		 * The membarrier system call requires each architecture
>  		 * to have a full memory barrier after updating