On 12/5/22 8:29 PM, Hou Tao wrote:
From: Hou Tao <houtao1@xxxxxxxxxx>
When there are batched freeing operations on a specific CPU, part of
the freed elements ((high_watermark - lower_watermark) / 2 + 1) will
be moved to waiting_for_gp list and the remaining part will be left in
free_by_rcu list and waits for the expiration of RCU-tasks-trace grace
period and the next invocation of free_bulk().
The change below LGTM. However, the above description seems not precise.
IIUC, free_by_rcu list => waiting_for_gp is controlled by whether
call_rcu_in_progress is true or not. If it is true, free_by_rcu list
will remain intact and not moving into waiting_for_gp list.
So it is not 'the remaining part will be left in free_by_rcu'.
It is all elements in free_by_rcu to waiting_for_gp or none.
So instead of invoking __alloc_percpu_gfp() or kmalloc_node() to
allocate a new object, in alloc_bulk() just check whether or not there
is freed element in free_by_rcu and reuse it if available.
Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
LGTM except the above suggestions.
Acked-by: Yonghong Song <yhs@xxxxxx>
---
kernel/bpf/memalloc.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index 8f0d65f2474a..7daf147bc8f6 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -171,9 +171,24 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
memcg = get_memcg(c);
old_memcg = set_active_memcg(memcg);
for (i = 0; i < cnt; i++) {
- obj = __alloc(c, node);
- if (!obj)
- break;
+ /*
+ * free_by_rcu is only manipulated by irq work refill_work().
+ * IRQ works on the same CPU are called sequentially, so it is
+ * safe to use __llist_del_first() here. If alloc_bulk() is
+ * invoked by the initial prefill, there will be no running
+ * irq work, so __llist_del_first() is fine as well.
+ *
+ * In most cases, objects on free_by_rcu are from the same CPU.
+ * If some objects come from other CPUs, it doesn't incur any
+ * harm because NUMA_NO_NODE means the preference for current
+ * numa node and it is not a guarantee.
+ */
+ obj = __llist_del_first(&c->free_by_rcu);
+ if (!obj) {
+ obj = __alloc(c, node);
+ if (!obj)
+ break;
+ }
if (IS_ENABLED(CONFIG_PREEMPT_RT))
/* In RT irq_work runs in per-cpu kthread, so disable
* interrupts to avoid preemption and interrupts and
