Messages in this thread | | | Date | Fri, 10 Dec 2021 11:29:31 -0500 | Subject | Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT | From | Waiman Long <> |
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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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