Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT
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From: Sebastian Andrzej Siewior <hidden>
Date: 2021-12-10 13:01:37
Also in:
linux-mm, lkml
On 2021-12-09 21:52:28 [-0500], Waiman Long wrote: …
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--- a/mm/memcontrol.c +++ b/mm/memcontrol.c
…
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@@ -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?
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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.
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- 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