Re: [RFC PATCH 2/3] mm/memcg: Add a local_lock_t for IRQ and TASK object.

[Waiman Long <longman@redhat.com>]

On 1/3/22 11:34, Sebastian Andrzej Siewior wrote:
> On 2021-12-23 16:38:35 [-0500], Waiman Long wrote:
>> AFAICS, stock_pcp is set to indicate that the stock_lock has been acquired.
>> Since stock_pcp, if set, should be the same as this_cpu_ptr(&memcg_stock),
>> it is a bit confusing to pass it to a function that also does a percpu
>> access to memcg_stock. Why don't you just pass a boolean, say, stock_locked
>> to indicate this instead. It will be more clear and less confusing.
> Something like this then?

Yes, that looks good to me.

Thanks,
Longman

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d2687d5ed544b..05fdc4801da6b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -261,8 +261,10 @@ bool mem_cgroup_kmem_disabled(void)
>   	return cgroup_memory_nokmem;
>   }
>   
> +struct memcg_stock_pcp;
>   static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
> -				      unsigned int nr_pages);
> +				      unsigned int nr_pages,
> +				      bool stock_lock_acquried);
>   
>   static void obj_cgroup_release(struct percpu_ref *ref)
>   {
> @@ -296,7 +298,7 @@ static void obj_cgroup_release(struct percpu_ref *ref)
>   	nr_pages = nr_bytes >> PAGE_SHIFT;
>   
>   	if (nr_pages)
> -		obj_cgroup_uncharge_pages(objcg, nr_pages);
> +		obj_cgroup_uncharge_pages(objcg, nr_pages, false);
>   
>   	spin_lock_irqsave(&css_set_lock, flags);
>   	list_del(&objcg->list);
> @@ -2120,26 +2122,40 @@ struct obj_stock {
>   };
>   
>   struct memcg_stock_pcp {
> +	/* Protects memcg_stock_pcp */
> +	local_lock_t stock_lock;
>   	struct mem_cgroup *cached; /* this never be root cgroup */
>   	unsigned int nr_pages;
> +#ifndef CONFIG_PREEMPT_RT
> +	/* Protects only task_obj */
> +	local_lock_t task_obj_lock;
>   	struct obj_stock task_obj;
> +#endif
>   	struct obj_stock irq_obj;
>   
>   	struct work_struct work;
>   	unsigned long flags;
>   #define FLUSHING_CACHED_CHARGE	0
>   };
> -static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> +static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
> +	.stock_lock = INIT_LOCAL_LOCK(stock_lock),
> +#ifndef CONFIG_PREEMPT_RT
> +	.task_obj_lock = INIT_LOCAL_LOCK(task_obj_lock),
> +#endif
> +};
>   static DEFINE_MUTEX(percpu_charge_mutex);
>   
>   #ifdef CONFIG_MEMCG_KMEM
> -static void drain_obj_stock(struct obj_stock *stock);
> +static struct obj_cgroup *drain_obj_stock(struct obj_stock *stock,
> +					  bool stock_lock_acquried);
>   static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>   				     struct mem_cgroup *root_memcg);
>   
>   #else
> -static inline void drain_obj_stock(struct obj_stock *stock)
> +static inline struct obj_cgroup *drain_obj_stock(struct obj_stock *stock,
> +						 bool stock_lock_acquried)
>   {
> +	return NULL;
>   }
>   static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>   				     struct mem_cgroup *root_memcg)
> @@ -2168,7 +2184,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>   	if (nr_pages > MEMCG_CHARGE_BATCH)
>   		return ret;
>   
> -	local_irq_save(flags);
> +	local_lock_irqsave(&memcg_stock.stock_lock, flags);
>   
>   	stock = this_cpu_ptr(&memcg_stock);
>   	if (memcg == stock->cached && stock->nr_pages >= nr_pages) {
> @@ -2176,7 +2192,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>   		ret = true;
>   	}
>   
> -	local_irq_restore(flags);
> +	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
>   
>   	return ret;
>   }
> @@ -2204,38 +2220,43 @@ static void drain_stock(struct memcg_stock_pcp *stock)
>   
>   static void drain_local_stock(struct work_struct *dummy)
>   {
> -	struct memcg_stock_pcp *stock;
> -	unsigned long flags;
> +	struct memcg_stock_pcp *stock_pcp;
> +	struct obj_cgroup *old;
>   
>   	/*
>   	 * The only protection from cpu hotplug (memcg_hotplug_cpu_dead) vs.
>   	 * drain_stock races is that we always operate on local CPU stock
>   	 * here with IRQ disabled
>   	 */
> -	local_irq_save(flags);
> +#ifndef CONFIG_PREEMPT_RT
> +	local_lock(&memcg_stock.task_obj_lock);
> +	old = drain_obj_stock(&this_cpu_ptr(&memcg_stock)->task_obj, NULL);
> +	local_unlock(&memcg_stock.task_obj_lock);
> +	if (old)
> +		obj_cgroup_put(old);
> +#endif
>   
> -	stock = this_cpu_ptr(&memcg_stock);
> -	drain_obj_stock(&stock->irq_obj);
> -	if (in_task())
> -		drain_obj_stock(&stock->task_obj);
> -	drain_stock(stock);
> -	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
> +	local_lock_irq(&memcg_stock.stock_lock);
> +	stock_pcp = this_cpu_ptr(&memcg_stock);
> +	old = drain_obj_stock(&stock_pcp->irq_obj, stock_pcp);
>   
> -	local_irq_restore(flags);
> +	drain_stock(stock_pcp);
> +	clear_bit(FLUSHING_CACHED_CHARGE, &stock_pcp->flags);
> +
> +	local_unlock_irq(&memcg_stock.stock_lock);
> +	if (old)
> +		obj_cgroup_put(old);
>   }
>   
>   /*
>    * Cache charges(val) to local per_cpu area.
>    * This will be consumed by consume_stock() function, later.
>    */
> -static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> +static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>   {
> -	struct memcg_stock_pcp *stock;
> -	unsigned long flags;
> +	struct memcg_stock_pcp *stock = this_cpu_ptr(&memcg_stock);
>   
> -	local_irq_save(flags);
> -
> -	stock = this_cpu_ptr(&memcg_stock);
> +	lockdep_assert_held(&stock->stock_lock);
>   	if (stock->cached != memcg) { /* reset if necessary */
>   		drain_stock(stock);
>   		css_get(&memcg->css);
> @@ -2245,8 +2266,20 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>   
>   	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
>   		drain_stock(stock);
> +}
>   
> -	local_irq_restore(flags);
> +static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages,
> +			 bool stock_lock_acquried)
> +{
> +	unsigned long flags;
> +
> +	if (stock_lock_acquried) {
> +		__refill_stock(memcg, nr_pages);
> +		return;
> +	}
> +	local_lock_irqsave(&memcg_stock.stock_lock, flags);
> +	__refill_stock(memcg, nr_pages);
> +	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
>   }
>   
>   /*
> @@ -2255,7 +2288,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>    */
>   static void drain_all_stock(struct mem_cgroup *root_memcg)
>   {
> -	int cpu, curcpu;
> +	int cpu;
>   
>   	/* If someone's already draining, avoid adding running more workers. */
>   	if (!mutex_trylock(&percpu_charge_mutex))
> @@ -2266,7 +2299,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
>   	 * as well as workers from this path always operate on the local
>   	 * per-cpu data. CPU up doesn't touch memcg_stock at all.
>   	 */
> -	curcpu = get_cpu();
> +	cpus_read_lock();
>   	for_each_online_cpu(cpu) {
>   		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
>   		struct mem_cgroup *memcg;
> @@ -2282,14 +2315,10 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
>   		rcu_read_unlock();
>   
>   		if (flush &&
> -		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> -			if (cpu == curcpu)
> -				drain_local_stock(&stock->work);
> -			else
> -				schedule_work_on(cpu, &stock->work);
> -		}
> +		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> +			schedule_work_on(cpu, &stock->work);
>   	}
> -	put_cpu();
> +	cpus_read_unlock();
>   	mutex_unlock(&percpu_charge_mutex);
>   }
>   
> @@ -2690,7 +2719,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>   
>   done_restock:
>   	if (batch > nr_pages)
> -		refill_stock(memcg, batch - nr_pages);
> +		refill_stock(memcg, batch - nr_pages, false);
>   
>   	/*
>   	 * If the hierarchy is above the normal consumption range, schedule
> @@ -2803,28 +2832,36 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
>    * can only be accessed after disabling interrupt. User context code can
>    * access interrupt object stock, but not vice versa.
>    */
> -static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
> +static inline struct obj_stock *get_obj_stock(unsigned long *pflags,
> +					      bool *stock_lock_acquried)
>   {
>   	struct memcg_stock_pcp *stock;
>   
> +#ifndef CONFIG_PREEMPT_RT
>   	if (likely(in_task())) {
>   		*pflags = 0UL;
> -		preempt_disable();
> +		*stock_lock_acquried = false;
> +		local_lock(&memcg_stock.task_obj_lock);
>   		stock = this_cpu_ptr(&memcg_stock);
>   		return &stock->task_obj;
>   	}
> -
> -	local_irq_save(*pflags);
> +#endif
> +	*stock_lock_acquried = true;
> +	local_lock_irqsave(&memcg_stock.stock_lock, *pflags);
>   	stock = this_cpu_ptr(&memcg_stock);
>   	return &stock->irq_obj;
>   }
>   
> -static inline void put_obj_stock(unsigned long flags)
> +static inline void put_obj_stock(unsigned long flags,
> +				 bool stock_lock_acquried)
>   {
> -	if (likely(in_task()))
> -		preempt_enable();
> -	else
> -		local_irq_restore(flags);
> +#ifndef CONFIG_PREEMPT_RT
> +	if (likely(!stock_lock_acquried)) {
> +		local_unlock(&memcg_stock.task_obj_lock);
> +		return;
> +	}
> +#endif
> +	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
>   }
>   
>   /*
> @@ -3002,7 +3039,8 @@ static void memcg_free_cache_id(int id)
>    * @nr_pages: number of pages to uncharge
>    */
>   static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
> -				      unsigned int nr_pages)
> +				      unsigned int nr_pages,
> +				      bool stock_lock_acquried)
>   {
>   	struct mem_cgroup *memcg;
>   
> @@ -3010,7 +3048,7 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
>   
>   	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
>   		page_counter_uncharge(&memcg->kmem, nr_pages);
> -	refill_stock(memcg, nr_pages);
> +	refill_stock(memcg, nr_pages, stock_lock_acquried);
>   
>   	css_put(&memcg->css);
>   }
> @@ -3084,7 +3122,7 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
>   		return;
>   
>   	objcg = __folio_objcg(folio);
> -	obj_cgroup_uncharge_pages(objcg, nr_pages);
> +	obj_cgroup_uncharge_pages(objcg, nr_pages, false);
>   	folio->memcg_data = 0;
>   	obj_cgroup_put(objcg);
>   }
> @@ -3092,17 +3130,21 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
>   void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
>   		     enum node_stat_item idx, int nr)
>   {
> +	bool stock_lock_acquried;
>   	unsigned long flags;
> -	struct obj_stock *stock = get_obj_stock(&flags);
> +	struct obj_cgroup *old = NULL;
> +	struct obj_stock *stock;
>   	int *bytes;
>   
> +	stock = get_obj_stock(&flags, &stock_lock_acquried);
>   	/*
>   	 * Save vmstat data in stock and skip vmstat array update unless
>   	 * accumulating over a page of vmstat data or when pgdat or idx
>   	 * changes.
>   	 */
>   	if (stock->cached_objcg != objcg) {
> -		drain_obj_stock(stock);
> +		old = drain_obj_stock(stock, stock_lock_acquried);
> +
>   		obj_cgroup_get(objcg);
>   		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
>   				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
> @@ -3146,38 +3188,43 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
>   	if (nr)
>   		mod_objcg_mlstate(objcg, pgdat, idx, nr);
>   
> -	put_obj_stock(flags);
> +	put_obj_stock(flags, stock_lock_acquried);
> +	if (old)
> +		obj_cgroup_put(old);
>   }
>   
>   static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
>   {
> +	bool stock_lock_acquried;
>   	unsigned long flags;
> -	struct obj_stock *stock = get_obj_stock(&flags);
> +	struct obj_stock *stock;
>   	bool ret = false;
>   
> +	stock = get_obj_stock(&flags, &stock_lock_acquried);
>   	if (objcg == stock->cached_objcg && stock->nr_bytes >= nr_bytes) {
>   		stock->nr_bytes -= nr_bytes;
>   		ret = true;
>   	}
>   
> -	put_obj_stock(flags);
> +	put_obj_stock(flags, stock_lock_acquried);
>   
>   	return ret;
>   }
>   
> -static void drain_obj_stock(struct obj_stock *stock)
> +static struct obj_cgroup *drain_obj_stock(struct obj_stock *stock,
> +					  bool stock_lock_acquried)
>   {
>   	struct obj_cgroup *old = stock->cached_objcg;
>   
>   	if (!old)
> -		return;
> +		return NULL;
>   
>   	if (stock->nr_bytes) {
>   		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>   		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
>   
>   		if (nr_pages)
> -			obj_cgroup_uncharge_pages(old, nr_pages);
> +			obj_cgroup_uncharge_pages(old, nr_pages, stock_lock_acquried);
>   
>   		/*
>   		 * The leftover is flushed to the centralized per-memcg value.
> @@ -3212,8 +3259,8 @@ static void drain_obj_stock(struct obj_stock *stock)
>   		stock->cached_pgdat = NULL;
>   	}
>   
> -	obj_cgroup_put(old);
>   	stock->cached_objcg = NULL;
> +	return old;
>   }
>   
>   static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> @@ -3221,11 +3268,13 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>   {
>   	struct mem_cgroup *memcg;
>   
> +#ifndef CONFIG_PREEMPT_RT
>   	if (in_task() && stock->task_obj.cached_objcg) {
>   		memcg = obj_cgroup_memcg(stock->task_obj.cached_objcg);
>   		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
>   			return true;
>   	}
> +#endif
>   	if (stock->irq_obj.cached_objcg) {
>   		memcg = obj_cgroup_memcg(stock->irq_obj.cached_objcg);
>   		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
> @@ -3238,12 +3287,15 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>   static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
>   			     bool allow_uncharge)
>   {
> +	bool stock_lock_acquried;
>   	unsigned long flags;
> -	struct obj_stock *stock = get_obj_stock(&flags);
> +	struct obj_stock *stock;
>   	unsigned int nr_pages = 0;
> +	struct obj_cgroup *old = NULL;
>   
> +	stock = get_obj_stock(&flags, &stock_lock_acquried);
>   	if (stock->cached_objcg != objcg) { /* reset if necessary */
> -		drain_obj_stock(stock);
> +		old = drain_obj_stock(stock, stock_lock_acquried);
>   		obj_cgroup_get(objcg);
>   		stock->cached_objcg = objcg;
>   		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
> @@ -3257,10 +3309,12 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
>   		stock->nr_bytes &= (PAGE_SIZE - 1);
>   	}
>   
> -	put_obj_stock(flags);
> +	put_obj_stock(flags, stock_lock_acquried);
> +	if (old)
> +		obj_cgroup_put(old);
>   
>   	if (nr_pages)
> -		obj_cgroup_uncharge_pages(objcg, nr_pages);
> +		obj_cgroup_uncharge_pages(objcg, nr_pages, false);
>   }
>   
>   int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
> @@ -7061,7 +7115,7 @@ void mem_cgroup_uncharge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages)
>   
>   	mod_memcg_state(memcg, MEMCG_SOCK, -nr_pages);
>   
> -	refill_stock(memcg, nr_pages);
> +	refill_stock(memcg, nr_pages, false);
>   }
>   
>   static int __init cgroup_memory(char *s)