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SubjectRe: [PATCH] kfree_rcu() should use the new kfree_bulk() interface for freeing rcu structures
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On 12/19/2017 12:41 PM, Jesper Dangaard Brouer wrote:
> On Tue, 19 Dec 2017 09:52:27 -0800 rao.shoaib@oracle.com wrote:
>
>> +/* Main RCU function that is called to free RCU structures */
>> +static void
>> +__rcu_bulk_free(struct rcu_head *head, rcu_callback_t func, int cpu, bool lazy)
>> +{
>> + unsigned long offset;
>> + void *ptr;
>> + struct rcu_bulk_free *rbf;
>> + struct rcu_bulk_free_container *rbfc = NULL;
>> +
>> + rbf = this_cpu_ptr(&cpu_rbf);
>> +
>> + if (unlikely(!rbf->rbf_init)) {
>> + spin_lock_init(&rbf->rbf_lock);
>> + rbf->rbf_cpu = smp_processor_id();
>> + rbf->rbf_init = true;
>> + }
>> +
>> + /* hold lock to protect against other cpu's */
>> + spin_lock_bh(&rbf->rbf_lock);
> I'm not sure this will be faster. Having to take a cross CPU lock here
> (+ BH-disable) could cause scaling issues. Hopefully this lock will
> not be used intensively by other CPUs, right?
>
>
> The current cost of __call_rcu() is a local_irq_save/restore (which is
> quite expensive, but doesn't cause cross CPU chatter).
>
> Later in __rcu_process_callbacks() we have a local_irq_save/restore for
> the entire list, plus a per object cost doing local_bh_disable/enable.
> And for each object we call __rcu_reclaim(), which in some cases
> directly call kfree().

As Paul has pointed out the lock is a per-cpu lock, the only reason for
another CPU to access this lock is if the rcu callbacks run on a
different CPU and there is nothing the code can do to avoid that but
that should be rare anyways.

>
>
> If I had to implement this: I would choose to do the optimization in
> __rcu_process_callbacks() create small on-call-stack ptr-array for
> kfree_bulk(). I would only optimize the case that call kfree()
> directly. In the while(list) loop I would defer calling
> __rcu_reclaim() for __is_kfree_rcu_offset(head->func), and instead add
> them to the ptr-array (and flush if the array is full in loop, and
> kfree_bulk flush after loop).
This is exactly what the current code is doing. It accumulates only the
calls made to
__kfree_rcu(head, offset) ==> kfree_call_rcu() ==> __bulk_free_rcu

__kfree_rcu has a check to make sure that an offset is being passed.

When a function pointer is passed the caller has to call
call_rcu/call_rcu_sched

Accumulating early avoids the individual cost of calling __call_rcu

Perhaps I do not understand your point.

Shoaib
>
> The real advantage of kfree_bulk() comes from amortizing the per kfree
> (behind-the-scenes) sync cost. There is an additional benefit, because
> objects comes from RCU and will hit a slower path in SLUB. The SLUB
> allocator is very fast for objects that gets recycled quickly (short
> lifetime), non-locked (cpu-local) double-cmpxchg. But slower for
> longer-lived/more-outstanding objects, as this hits a slower code-path,
> fully locked (cross-cpu) double-cmpxchg.
>
>> +
>> + rbfc = rbf->rbf_container;
>> +
>> + if (rbfc == NULL) {
>> + if (rbf->rbf_cached_container == NULL) {
>> + rbf->rbf_container =
>> + kmalloc(sizeof(struct rcu_bulk_free_container),
>> + GFP_ATOMIC);
>> + rbf->rbf_container->rbfc_rbf = rbf;
>> + } else {
>> + rbf->rbf_container = rbf->rbf_cached_container;
>> + rbf->rbf_container->rbfc_rbf = rbf;
>> + cmpxchg(&rbf->rbf_cached_container,
>> + rbf->rbf_cached_container, NULL);
>> + }
>> +
>> + if (unlikely(rbf->rbf_container == NULL)) {
>> +
>> + /* Memory allocation failed maintain a list */
>> +
>> + head->func = (void *)func;
>> + head->next = rbf->rbf_list_head;
>> + rbf->rbf_list_head = head;
>> + rbf->rbf_list_size++;
>> + if (rbf->rbf_list_size == RCU_MAX_ACCUMULATE_SIZE)
>> + __rcu_bulk_schedule_list(rbf);
>> +
>> + goto done;
>> + }
>> +
>> + rbfc = rbf->rbf_container;
>> + rbfc->rbfc_entries = 0;
>> +
>> + if (rbf->rbf_list_head != NULL)
>> + __rcu_bulk_schedule_list(rbf);
>> + }
>> +
>> + offset = (unsigned long)func;
>> + ptr = (void *)head - offset;
>> +
>> + rbfc->rbfc_data[rbfc->rbfc_entries++] = ptr;
>> + if (rbfc->rbfc_entries == RCU_MAX_ACCUMULATE_SIZE) {
>> +
>> + WRITE_ONCE(rbf->rbf_container, NULL);
>> + spin_unlock_bh(&rbf->rbf_lock);
>> + call_rcu(&rbfc->rbfc_rcu, __rcu_bulk_free_impl);
>> + return;
>> + }
>> +
>> +done:
>> + if (!rbf->rbf_monitor) {
>> +
>> + call_rcu(&rbf->rbf_rcu, __rcu_bulk_free_monitor);
>> + rbf->rbf_monitor = true;
>> + }
>> +
>> + spin_unlock_bh(&rbf->rbf_lock);
>> +}
>

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