Messages in this thread | | | Date | Fri, 10 Dec 2021 14:01:32 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT |
| |
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?
> 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.
> > - local_irq_save(*pflags); > - stock = this_cpu_ptr(&memcg_stock); > - return &stock->irq_obj; > + local_lock_irqsave(&memcg_stock.lock, *pflags); > + return this_cpu_ptr(&memcg_stock.irq_obj); > } > > static inline void put_obj_stock(unsigned long flags) > { > - if (likely(in_task())) > + if (likely(in_task()) && !IS_ENABLED(CONFIG_PREEMPT_RT)) > preempt_enable(); > else > - local_irq_restore(flags); > + local_unlock_irqrestore(&memcg_stock.lock, flags); > } > > /*
Sebastian
| |