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SubjectRe: [PATCH] kfree_rcu() should use the new kfree_bulk() interface for freeing rcu structures
On Tue, Dec 19, 2017 at 09:41:58PM +0100, Jesper Dangaard Brouer wrote:
> 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.

Something like this ... (compile tested only)

Considerably less code; Rao, what do you think?

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 59c471de342a..5ac4ed077233 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -174,20 +174,19 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
}
#endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */

-void kfree(const void *);
-
/*
* Reclaim the specified callback, either by invoking it (non-lazy case)
* or freeing it directly (lazy case). Return true if lazy, false otherwise.
*/
-static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
+static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head, void **kfree,
+ unsigned int *idx)
{
unsigned long offset = (unsigned long)head->func;

rcu_lock_acquire(&rcu_callback_map);
if (__is_kfree_rcu_offset(offset)) {
RCU_TRACE(trace_rcu_invoke_kfree_callback(rn, head, offset);)
- kfree((void *)head - offset);
+ kfree[*idx++] = (void *)head - offset;
rcu_lock_release(&rcu_callback_map);
return true;
} else {
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f9c0ca2ccf0c..7e13979b4697 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2725,6 +2725,8 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
struct rcu_head *rhp;
struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
long bl, count;
+ void *to_free[16];
+ unsigned int to_free_idx = 0;

/* If no callbacks are ready, just return. */
if (!rcu_segcblist_ready_cbs(&rdp->cblist)) {
@@ -2755,8 +2757,10 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
rhp = rcu_cblist_dequeue(&rcl);
for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
debug_rcu_head_unqueue(rhp);
- if (__rcu_reclaim(rsp->name, rhp))
+ if (__rcu_reclaim(rsp->name, rhp, to_free, &to_free_idx))
rcu_cblist_dequeued_lazy(&rcl);
+ if (to_free_idx == 16)
+ kfree_bulk(16, to_free);
/*
* Stop only if limit reached and CPU has something to do.
* Note: The rcl structure counts down from zero.
@@ -2766,6 +2770,8 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
(!is_idle_task(current) && !rcu_is_callbacks_kthread())))
break;
}
+ if (to_free_idx)
+ kfree_bulk(to_free_idx, to_free);

local_irq_save(flags);
count = -rcl.len;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index db85ca3975f1..4127be06759b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2189,6 +2189,8 @@ static int rcu_nocb_kthread(void *arg)
struct rcu_head *next;
struct rcu_head **tail;
struct rcu_data *rdp = arg;
+ void *to_free[16];
+ unsigned int to_free_idx = 0;

/* Each pass through this loop invokes one batch of callbacks */
for (;;) {
@@ -2226,13 +2228,18 @@ static int rcu_nocb_kthread(void *arg)
}
debug_rcu_head_unqueue(list);
local_bh_disable();
- if (__rcu_reclaim(rdp->rsp->name, list))
+ if (__rcu_reclaim(rdp->rsp->name, list, to_free,
+ &to_free_idx))
cl++;
c++;
+ if (to_free_idx == 16)
+ kfree_bulk(16, to_free);
local_bh_enable();
cond_resched_rcu_qs();
list = next;
}
+ if (to_free_idx)
+ kfree_bulk(to_free_idx, to_free);
trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
smp_mb__before_atomic(); /* _add after CB invocation. */
atomic_long_add(-c, &rdp->nocb_q_count);
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