Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Feb 11, 2022 at 11:35:36PM +0100, Sebastian Andrzej Siewior wrote:
> The per-CPU counter are modified with the non-atomic modifier. The
> consistency is ensured by disabling interrupts for the update.
> On non PREEMPT_RT configuration this works because acquiring a
> spinlock_t typed lock with the _irq() suffix disables interrupts. On
> PREEMPT_RT configurations the RMW operation can be interrupted.
> 
> Another problem is that mem_cgroup_swapout() expects to be invoked with
> disabled interrupts because the caller has to acquire a spinlock_t which
> is acquired with disabled interrupts. Since spinlock_t never disables
> interrupts on PREEMPT_RT the interrupts are never disabled at this
> point.
> 
> The code is never called from in_irq() context on PREEMPT_RT therefore
> disabling preemption during the update is sufficient on PREEMPT_RT.
> The sections which explicitly disable interrupts can remain on
> PREEMPT_RT because the sections remain short and they don't involve
> sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT).
> 
> Disable preemption during update of the per-CPU variables which do not
> explicitly disable interrupts.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  mm/memcontrol.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c1caa662946dc..466466f285cea 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
>  	memcg = pn->memcg;
>  
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_disable();
>  	/* Update memcg */
>  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
>  
> @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
>  
>  	memcg_rstat_updated(memcg, val);
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_enable();
>  }

I notice you didn't annoate __mod_memcg_state(). I suppose that is
because it's called with explicit local_irq_disable(), and that
disables preemption on rt? And you only need another preempt_disable()
for stacks that rely on coming from spin_lock_irq(save)?

That makes sense, but it's difficult to maintain. It'll easily break
if somebody adds more memory accounting sites that may also rely on an
irq-disabled spinlock somewhere.

So better to make this an unconditional locking protocol:

static void memcg_stats_lock(void)
{
#ifdef CONFIG_PREEMPT_RT
	preempt_disable();
#else
	VM_BUG_ON(!irqs_disabled());
#endif
}

static void memcg_stats_unlock(void)
{
#ifdef CONFIG_PREEMPT_RT
	preempt_enable();
#endif
}

and always use these around the counter updates.