On Tue, 19 Dec 2017 09:52:27 -0800 rao.shoaib@xxxxxxxxxx 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().
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).
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);
> +}
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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