Re: [PATCH 11/24] rcu/tree: Maintain separate array for vmalloc ptrs

Paul E. McKenney
2020-05-04T02:20:06Z

On Sun, May 03, 2020 at 08:58:58PM -0400, Joel Fernandes wrote:
> On Sun, May 03, 2020 at 05:20:32PM -0700, Paul E. McKenney wrote:
> > On Sun, May 03, 2020 at 07:42:50PM -0400, Joel Fernandes wrote:
> > > 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?
> >
> > It looks like rcu_callback_map was been added by 77a40f97030 ("rcu:
> > Remove kfree_rcu() special casing and lazy-callback handling"). Which
> > was less than a year ago. ;-)
>
> I think that's just git blame falsely looking at moved code instead of the
> original code.
>
> It was actually the following commit. I think you were trying to detect
> blocking and context-switching within an RCU callback. Since kfree_rcu() does
> not have RCU callback functions, may be we can just remove it?
>
> commit 24ef659a857c3cba40b64ea51ea4fce8d2fb7bbc
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date: Mon Oct 28 09:22:24 2013 -0700
>
> rcu: Provide better diagnostics for blocking in RCU callback functions
>
> Currently blocking in an RCU callback function will result in
> "scheduling while atomic", which could be triggered for any number
> of reasons. To aid debugging, this patch introduces a rcu_callback_map
> that is used to tie the inappropriate voluntary context switch back
> to the fact that the function is being invoked from within a callback.

Right you are!

I was fooled as you say by the code movement. I was searching for
rcu_callback_map in kernel/rcu/tree.c rather than using "git grep"
or similar.

So I took my own advice and added a comment. ;-)

Thanx, Paul

> thanks,
>
> - Joel
>
>
> >
> > Hmmm... This would be a good way to allow lockdep to tell you that you
> > are running within an RCU callback on the one hand are are reclaiming
> > due to kfree_rcu() on the other. Was that the intent? If so, a comment
> > seems necessary.
> >
> > Thanx, Paul
> >
> > > 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
> > > > >