On 12/19/2017 12:41 PM, Jesper Dangaard Brouer wrote:
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().
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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