Re: [PATCH bpf-next v4] bpf: Initialize same number of free nodes for each pcpu_freelist

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On 11/11/2022 11:53 AM, wuqiang wrote:
On 2022/11/10 20:21, Xu Kuohai wrote:
pcpu_freelist_populate() initializes nr_elems / num_possible_cpus() + 1
free nodes for some CPUs, and then possibly one CPU with fewer nodes,
followed by remaining cpus with 0 nodes. For example, when nr_elems == 256
and num_possible_cpus() == 32, CPU 0~27 each gets 9 free nodes, CPU 28 gets
4 free nodes, CPU 29~31 get 0 free nodes, while in fact each CPU should get
8 nodes equally.

This patch initializes nr_elems / num_possible_cpus() free nodes for each
CPU firstly, then allocates the remaining free nodes by one for each CPU
until no free nodes left.

Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
---
v4: Remove unneeded min()
v3: Simplify code as suggested by Andrii
v2: Update commit message and add Yonghong's ack
---
  kernel/bpf/percpu_freelist.c | 26 +++++++++++++-------------
  1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
index b6e7f5c5b9ab..27f2c4aff623 100644
--- a/kernel/bpf/percpu_freelist.c
+++ b/kernel/bpf/percpu_freelist.c
@@ -100,22 +100,22 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
                  u32 nr_elems)
  {
      struct pcpu_freelist_head *head;
-    int i, cpu, pcpu_entries;
+    unsigned int cpu, cpu_idx, i, j, n, m;
-    pcpu_entries = nr_elems / num_possible_cpus() + 1;
-    i = 0;
+    n = nr_elems / num_possible_cpus();
+    m = nr_elems % num_possible_cpus();
+
+    cpu_idx = 0;
      for_each_possible_cpu(cpu) {
-again:
-        head = per_cpu_ptr(s->freelist, cpu);
-        /* No locking required as this is not visible yet. */
-        pcpu_freelist_push_node(head, buf);
-        i++;
-        buf += elem_size;
-        if (i == nr_elems)
-            break;
-        if (i % pcpu_entries)
-            goto again;
+        j = n + (cpu_idx < m ? 1 : 0);
+        for (i = 0; i < j; i++) {
+            head = per_cpu_ptr(s->freelist, cpu);

Better move it out of "i-loop",

OK, will do

and rename "j" to a meaningful name to avoid
possible misuse.

The loop is short enough to be readable and "j" is not used elsewhere, so I
think it's good to keep the name simple.

+            /* No locking required as this is not visible yet. */
+            pcpu_freelist_push_node(head, buf);
+            buf += elem_size;
+        }
+        cpu_idx++;
      }
  }

.




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