Re: [PATCH 2/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Vlastimil Babka <vbabka@suse.cz>]

On 1/25/22 17:43, 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>

So it's like c68ed7945701 ("mm/vmstat: protect per cpu variables with
preempt disable on RT") but we still don't want a wrapper of those
constructs so they don't spread further, right :)

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/memcontrol.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 36d27db673ca9..3d1b7cdd83db0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -667,6 +667,8 @@ 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);
>  
> @@ -674,6 +676,8 @@ 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();
>  }
>  
>  /**
> @@ -756,8 +760,12 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
>  	if (mem_cgroup_disabled())
>  		return;
>  
> +	if (IS_ENABLED(PREEMPT_RT))
> +		preempt_disable();
>  	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
>  	memcg_rstat_updated(memcg, count);
> +	if (IS_ENABLED(PREEMPT_RT))
> +		preempt_enable();
>  }
>  
>  static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
> @@ -7194,9 +7202,18 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	 * i_pages lock which is taken with interrupts-off. It is
>  	 * important here to have the interrupts disabled because it is the
>  	 * only synchronisation we have for updating the per-CPU variables.
> +	 * On PREEMPT_RT interrupts are never disabled and the updates to per-CPU
> +	 * variables are synchronised by keeping preemption disabled.
>  	 */
> -	VM_BUG_ON(!irqs_disabled());
> -	mem_cgroup_charge_statistics(memcg, -nr_entries);
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		VM_BUG_ON(!irqs_disabled());
> +		mem_cgroup_charge_statistics(memcg, -nr_entries);
> +	} else {
> +		preempt_disable();
> +		mem_cgroup_charge_statistics(memcg, -nr_entries);
> +		preempt_enable();
> +	}
> +
>  	memcg_check_events(memcg, page_to_nid(page));
>  
>  	css_put(&memcg->css);