On Fri, May 01, 2020 at 02:37:53PM -0700, Paul E. McKenney wrote:
[...]
> > @@ -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...
I believe the rcu_callback_map usage above follows a similar pattern from old
code where the rcu_callback_map is acquired before doing the kfree.
static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
{
rcu_callback_t f;
unsigned long offset = (unsigned long)head->func;
rcu_lock_acquire(&rcu_callback_map);
if (__is_kfree_rcu_offset(offset)) {
trace_rcu_invoke_kfree_callback(rn, head, offset);
kfree((void *)head - offset);
rcu_lock_release(&rcu_callback_map);
So when kfree_rcu() was rewritten, the rcu_lock_acquire() of rcu_callback_map
got carried.
I believe it is for detecting recursion where we possibly try to free
RCU-held memory while already freeing memory. Or was there anoher purpose of
the rcu_callback_map?
thanks,
- Joel
> > 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
> >
