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Date:   Fri, 1 May 2020 14:37:53 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 11/24] rcu/tree: Maintain separate array for vmalloc ptrs

On Tue, Apr 28, 2020 at 10:58:50PM +0200, Uladzislau Rezki (Sony) wrote:
> To do so we use an array of common kvfree_rcu_bulk_data
> structure. It consists of two elements, index number 0
> corresponds to SLAB ptrs., whereas vmalloc pointers can
> be accessed by using index number 1.
> 
> The reason of not mixing pointers is to have an easy way
> to to distinguish them.
> 
> It is also the preparation patch for head-less objects
> support. When an object is head-less we can not queue
> it into any list, instead a pointer is placed directly
> into an array.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Co-developed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree.c | 172 +++++++++++++++++++++++++++++-----------------
>  1 file changed, 109 insertions(+), 63 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d8975819b1c9..7983926af95b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -57,6 +57,7 @@
>  #include <linux/slab.h>
>  #include <linux/sched/isolation.h>
>  #include <linux/sched/clock.h>
> +#include <linux/mm.h>
>  #include "../time/tick-internal.h"
>  
>  #include "tree.h"
> @@ -2857,44 +2858,44 @@ EXPORT_SYMBOL_GPL(call_rcu);
>  #define KFREE_N_BATCHES 2
>  
>  /**
> - * struct kfree_rcu_bulk_data - single block to store kfree_rcu() pointers
> + * struct kvfree_rcu_bulk_data - single block to store kvfree_rcu() pointers
>   * @nr_records: Number of active pointers in the array
> - * @records: Array of the kfree_rcu() pointers
>   * @next: Next bulk object in the block chain
> + * @records: Array of the kvfree_rcu() pointers
>   */
> -struct kfree_rcu_bulk_data {
> +struct kvfree_rcu_bulk_data {
>  	unsigned long nr_records;
> -	struct kfree_rcu_bulk_data *next;
> +	struct kvfree_rcu_bulk_data *next;
>  	void *records[];
>  };
>  
>  /*
>   * This macro defines how many entries the "records" array
>   * will contain. It is based on the fact that the size of
> - * kfree_rcu_bulk_data structure becomes exactly one page.
> + * kvfree_rcu_bulk_data structure becomes exactly one page.
>   */
> -#define KFREE_BULK_MAX_ENTR \
> -	((PAGE_SIZE - sizeof(struct kfree_rcu_bulk_data)) / sizeof(void *))
> +#define KVFREE_BULK_MAX_ENTR \
> +	((PAGE_SIZE - sizeof(struct kvfree_rcu_bulk_data)) / sizeof(void *))
>  
>  /**
>   * struct kfree_rcu_cpu_work - single batch of kfree_rcu() requests
>   * @rcu_work: Let queue_rcu_work() invoke workqueue handler after grace period
>   * @head_free: List of kfree_rcu() objects waiting for a grace period
> - * @bhead_free: Bulk-List of kfree_rcu() objects waiting for a grace period
> + * @bkvhead_free: Bulk-List of kvfree_rcu() objects waiting for a grace period
>   * @krcp: Pointer to @kfree_rcu_cpu structure
>   */
>  
>  struct kfree_rcu_cpu_work {
>  	struct rcu_work rcu_work;
>  	struct rcu_head *head_free;
> -	struct kfree_rcu_bulk_data *bhead_free;
> +	struct kvfree_rcu_bulk_data *bkvhead_free[2];
>  	struct kfree_rcu_cpu *krcp;
>  };
>  
>  /**
>   * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
>   * @head: List of kfree_rcu() objects not yet waiting for a grace period
> - * @bhead: Bulk-List of kfree_rcu() objects not yet waiting for a grace period
> + * @bkvhead: Bulk-List of kvfree_rcu() objects not yet waiting for a grace period
>   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
>   * @lock: Synchronize access to this structure
>   * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
> @@ -2908,7 +2909,7 @@ struct kfree_rcu_cpu_work {
>   */
>  struct kfree_rcu_cpu {
>  	struct rcu_head *head;
> -	struct kfree_rcu_bulk_data *bhead;
> +	struct kvfree_rcu_bulk_data *bkvhead[2];
>  	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
>  	raw_spinlock_t lock;
>  	struct delayed_work monitor_work;
> @@ -2932,7 +2933,7 @@ static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
>  };
>  
>  static __always_inline void
> -debug_rcu_bhead_unqueue(struct kfree_rcu_bulk_data *bhead)
> +debug_rcu_bhead_unqueue(struct kvfree_rcu_bulk_data *bhead)
>  {
>  #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
>  	int i;
> @@ -2961,20 +2962,20 @@ krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
>  	local_irq_restore(flags);
>  }
>  
> -static inline struct kfree_rcu_bulk_data *
> +static inline struct kvfree_rcu_bulk_data *
>  get_cached_bnode(struct kfree_rcu_cpu *krcp)
>  {
>  	if (!krcp->nr_bkv_objs)
>  		return NULL;
>  
>  	krcp->nr_bkv_objs--;
> -	return (struct kfree_rcu_bulk_data *)
> +	return (struct kvfree_rcu_bulk_data *)
>  		llist_del_first(&krcp->bkvcache);
>  }
>  
>  static inline bool
>  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> -	struct kfree_rcu_bulk_data *bnode)
> +	struct kvfree_rcu_bulk_data *bnode)
>  {
>  	/* Check the limit. */
>  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> @@ -2993,41 +2994,73 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
>  static void kfree_rcu_work(struct work_struct *work)
>  {
>  	unsigned long flags;
> +	struct kvfree_rcu_bulk_data *bkhead, *bvhead, *bnext;
>  	struct rcu_head *head, *next;
> -	struct kfree_rcu_bulk_data *bhead, *bnext;
>  	struct kfree_rcu_cpu *krcp;
>  	struct kfree_rcu_cpu_work *krwp;
> +	int i;
>  
>  	krwp = container_of(to_rcu_work(work),
>  			    struct kfree_rcu_cpu_work, rcu_work);
>  	krcp = krwp->krcp;
> +
>  	raw_spin_lock_irqsave(&krcp->lock, flags);
> +	/* Channel 1. */
> +	bkhead = krwp->bkvhead_free[0];
> +	krwp->bkvhead_free[0] = NULL;
> +
> +	/* Channel 2. */
> +	bvhead = krwp->bkvhead_free[1];
> +	krwp->bkvhead_free[1] = NULL;
> +
> +	/* Channel 3. */
>  	head = krwp->head_free;
>  	krwp->head_free = NULL;
> -	bhead = krwp->bhead_free;
> -	krwp->bhead_free = NULL;
>  	raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  
> -	/* "bhead" is now private, so traverse locklessly. */
> -	for (; bhead; bhead = bnext) {
> -		bnext = bhead->next;
> -
> -		debug_rcu_bhead_unqueue(bhead);
> +	/* kmalloc()/kfree() channel. */
> +	for (; bkhead; bkhead = bnext) {
> +		bnext = bkhead->next;
> +		debug_rcu_bhead_unqueue(bkhead);
>  
>  		rcu_lock_acquire(&rcu_callback_map);

Given that rcu_lock_acquire() only affects lockdep, I have to ask exactly
what concurrency design you are using here...

>  		trace_rcu_invoke_kfree_bulk_callback(rcu_state.name,
> -			bhead->nr_records, bhead->records);
> +			bkhead->nr_records, bkhead->records);
> +
> +		kfree_bulk(bkhead->nr_records, bkhead->records);
> +		rcu_lock_release(&rcu_callback_map);
> +
> +		krcp = krc_this_cpu_lock(&flags);
> +		if (put_cached_bnode(krcp, bkhead))
> +			bkhead = NULL;
> +		krc_this_cpu_unlock(krcp, flags);
> +
> +		if (bkhead)
> +			free_page((unsigned long) bkhead);
> +
> +		cond_resched_tasks_rcu_qs();
> +	}
> +
> +	/* vmalloc()/vfree() channel. */
> +	for (; bvhead; bvhead = bnext) {
> +		bnext = bvhead->next;
> +		debug_rcu_bhead_unqueue(bvhead);
>  
> -		kfree_bulk(bhead->nr_records, bhead->records);
> +		rcu_lock_acquire(&rcu_callback_map);

And the same here.

> +		for (i = 0; i < bvhead->nr_records; i++) {
> +			trace_rcu_invoke_kfree_callback(rcu_state.name,
> +				(struct rcu_head *) bvhead->records[i], 0);
> +			vfree(bvhead->records[i]);
> +		}
>  		rcu_lock_release(&rcu_callback_map);
>  
>  		krcp = krc_this_cpu_lock(&flags);
> -		if (put_cached_bnode(krcp, bhead))
> -			bhead = NULL;
> +		if (put_cached_bnode(krcp, bvhead))
> +			bvhead = NULL;
>  		krc_this_cpu_unlock(krcp, flags);
>  
> -		if (bhead)
> -			free_page((unsigned long) bhead);
> +		if (bvhead)
> +			free_page((unsigned long) bvhead);
>  
>  		cond_resched_tasks_rcu_qs();
>  	}
> @@ -3047,7 +3080,7 @@ static void kfree_rcu_work(struct work_struct *work)
>  		trace_rcu_invoke_kfree_callback(rcu_state.name, head, offset);
>  
>  		if (!WARN_ON_ONCE(!__is_kfree_rcu_offset(offset)))
> -			kfree(ptr);
> +			kvfree(ptr);
>  
>  		rcu_lock_release(&rcu_callback_map);
>  		cond_resched_tasks_rcu_qs();
> @@ -3072,21 +3105,34 @@ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
>  		krwp = &(krcp->krw_arr[i]);
>  
>  		/*
> -		 * Try to detach bhead or head and attach it over any
> +		 * Try to detach bkvhead or head and attach it over any
>  		 * available corresponding free channel. It can be that
>  		 * a previous RCU batch is in progress, it means that
>  		 * immediately to queue another one is not possible so
>  		 * return false to tell caller to retry.
>  		 */
> -		if ((krcp->bhead && !krwp->bhead_free) ||
> +		if ((krcp->bkvhead[0] && !krwp->bkvhead_free[0]) ||
> +			(krcp->bkvhead[1] && !krwp->bkvhead_free[1]) ||
>  				(krcp->head && !krwp->head_free)) {
> -			/* Channel 1. */
> -			if (!krwp->bhead_free) {
> -				krwp->bhead_free = krcp->bhead;
> -				krcp->bhead = NULL;
> +			/*
> +			 * Channel 1 corresponds to SLAB ptrs.
> +			 */
> +			if (!krwp->bkvhead_free[0]) {
> +				krwp->bkvhead_free[0] = krcp->bkvhead[0];
> +				krcp->bkvhead[0] = NULL;
>  			}
>  
> -			/* Channel 2. */
> +			/*
> +			 * Channel 2 corresponds to vmalloc ptrs.
> +			 */
> +			if (!krwp->bkvhead_free[1]) {
> +				krwp->bkvhead_free[1] = krcp->bkvhead[1];
> +				krcp->bkvhead[1] = NULL;
> +			}

Why not a "for" loop here?  Duplicate code is most certainly not what
we want, as it can cause all sorts of trouble down the road.

							Thanx, Paul

> +			/*
> +			 * Channel 3 corresponds to emergency path.
> +			 */
>  			if (!krwp->head_free) {
>  				krwp->head_free = krcp->head;
>  				krcp->head = NULL;
> @@ -3095,16 +3141,17 @@ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
>  			WRITE_ONCE(krcp->count, 0);
>  
>  			/*
> -			 * One work is per one batch, so there are two "free channels",
> -			 * "bhead_free" and "head_free" the batch can handle. It can be
> -			 * that the work is in the pending state when two channels have
> -			 * been detached following each other, one by one.
> +			 * One work is per one batch, so there are three
> +			 * "free channels", the batch can handle. It can
> +			 * be that the work is in the pending state when
> +			 * channels have been detached following by each
> +			 * other.
>  			 */
>  			queue_rcu_work(system_wq, &krwp->rcu_work);
>  		}
>  
>  		/* Repeat if any "free" corresponding channel is still busy. */
> -		if (krcp->bhead || krcp->head)
> +		if (krcp->bkvhead[0] || krcp->bkvhead[1] || krcp->head)
>  			repeat = true;
>  	}
>  
> @@ -3146,23 +3193,22 @@ static void kfree_rcu_monitor(struct work_struct *work)
>  }
>  
>  static inline bool
> -kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
> -	struct rcu_head *head, rcu_callback_t func)
> +kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
>  {
> -	struct kfree_rcu_bulk_data *bnode;
> +	struct kvfree_rcu_bulk_data *bnode;
> +	int idx;
>  
>  	if (unlikely(!krcp->initialized))
>  		return false;
>  
>  	lockdep_assert_held(&krcp->lock);
> +	idx = !!is_vmalloc_addr(ptr);
>  
>  	/* Check if a new block is required. */
> -	if (!krcp->bhead ||
> -			krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) {
> +	if (!krcp->bkvhead[idx] ||
> +			krcp->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
>  		bnode = get_cached_bnode(krcp);
>  		if (!bnode) {
> -			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
> -
>  			/*
>  			 * To keep this path working on raw non-preemptible
>  			 * sections, prevent the optional entry into the
> @@ -3175,7 +3221,7 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
>  			if (IS_ENABLED(CONFIG_PREEMPT_RT))
>  				return false;
>  
> -			bnode = (struct kfree_rcu_bulk_data *)
> +			bnode = (struct kvfree_rcu_bulk_data *)
>  				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
>  		}
>  
> @@ -3185,30 +3231,30 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
>  
>  		/* Initialize the new block. */
>  		bnode->nr_records = 0;
> -		bnode->next = krcp->bhead;
> +		bnode->next = krcp->bkvhead[idx];
>  
>  		/* Attach it to the head. */
> -		krcp->bhead = bnode;
> +		krcp->bkvhead[idx] = bnode;
>  	}
>  
>  	/* Finally insert. */
> -	krcp->bhead->records[krcp->bhead->nr_records++] =
> -		(void *) head - (unsigned long) func;
> +	krcp->bkvhead[idx]->records
> +		[krcp->bkvhead[idx]->nr_records++] = ptr;
>  
>  	return true;
>  }
>  
>  /*
> - * Queue a request for lazy invocation of kfree_bulk()/kfree() after a grace
> - * period. Please note there are two paths are maintained, one is the main one
> - * that uses kfree_bulk() interface and second one is emergency one, that is
> - * used only when the main path can not be maintained temporary, due to memory
> - * pressure.
> + * Queue a request for lazy invocation of appropriate free routine after a
> + * grace period. Please note there are three paths are maintained, two are the
> + * main ones that use array of pointers interface and third one is emergency
> + * one, that is used only when the main path can not be maintained temporary,
> + * due to memory pressure.
>   *
>   * Each kfree_call_rcu() request is added to a batch. The batch will be drained
>   * every KFREE_DRAIN_JIFFIES number of jiffies. All the objects in the batch will
>   * be free'd in workqueue context. This allows us to: batch requests together to
> - * reduce the number of grace periods during heavy kfree_rcu() load.
> + * reduce the number of grace periods during heavy kfree_rcu()/kvfree_rcu() load.
>   */
>  void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  {
> @@ -3231,7 +3277,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  	 * Under high memory pressure GFP_NOWAIT can fail,
>  	 * in that case the emergency path is maintained.
>  	 */
> -	if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp, head, func))) {
> +	if (unlikely(!kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr))) {
>  		head->func = func;
>  		head->next = krcp->head;
>  		krcp->head = head;
> @@ -4212,7 +4258,7 @@ static void __init kfree_rcu_batch_init(void)
>  
>  	for_each_possible_cpu(cpu) {
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> -		struct kfree_rcu_bulk_data *bnode;
> +		struct kvfree_rcu_bulk_data *bnode;
>  
>  		for (i = 0; i < KFREE_N_BATCHES; i++) {
>  			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
> @@ -4220,7 +4266,7 @@ static void __init kfree_rcu_batch_init(void)
>  		}
>  
>  		for (i = 0; i < rcu_min_cached_objs; i++) {
> -			bnode = (struct kfree_rcu_bulk_data *)
> +			bnode = (struct kvfree_rcu_bulk_data *)
>  				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
>  
>  			if (bnode)
> -- 
> 2.20.1
> 

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