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SubjectRe: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT
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On 12/10/21 08:01, Sebastian Andrzej Siewior wrote:
> On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
> …
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
> …
>> @@ -2210,7 +2211,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>> struct memcg_stock_pcp *stock;
>> unsigned long flags;
>>
>> - local_irq_save(flags);
>> + local_lock_irqsave(&memcg_stock.lock, flags);
> Why is this one using the lock? It isn't accessing irq_obj, right?
Well, the lock isn't just for irq_obj. It protects the whole memcg_stock
structure which include irq_obj. Sometimes, data in irq_obj (or
task_obj) will get transfer to nr_pages and vice versa. So it is easier
to use one single lock for the whole thing.
>
>> stock = this_cpu_ptr(&memcg_stock);
>> if (stock->cached != memcg) { /* reset if necessary */
>> @@ -2779,29 +2780,28 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
>> * which is cheap in non-preempt kernel. The interrupt context object stock
>> * can only be accessed after disabling interrupt. User context code can
>> * access interrupt object stock, but not vice versa.
>> + *
>> + * This task and interrupt context optimization is disabled for PREEMPT_RT
>> + * as there is no performance gain in this case.
>> */
>> static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
>> {
>> - struct memcg_stock_pcp *stock;
>> -
>> - if (likely(in_task())) {
>> + if (likely(in_task()) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
>> *pflags = 0UL;
>> preempt_disable();
>> - stock = this_cpu_ptr(&memcg_stock);
>> - return &stock->task_obj;
>> + return this_cpu_ptr(&memcg_stock.task_obj);
>> }
> We usually add the local_lock_t to the object it protects, struct
> obj_stock it this case.
> That would give you two different locks (instead of one) so you wouldn't
> have to use preempt_disable() to avoid lockdep's complains. Also it
> would warn you if you happen to use that obj_stock in !in_task() which
> is isn't possible now.
> The only downside would be that drain_local_stock() needs to acquire two
> locks.
>
As said above, having separate locks will complicate the interaction
between irq_obj and the broader memcg_stock fields. Besides throughput
is a less important matrix for PREEMPT_RT, so I am not trying to
optimize throughput performance for PREEMPT_RT here.

Cheers,
Longman

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