Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT

[Waiman Long]

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