linux-kernel - [PATCH bpf-next v5] bpf: Initialize same number of free nodes for each pcpu

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Date:   Fri, 11 Nov 2022 08:22:58 -0500
From:   Xu Kuohai <xukuohai@...weicloud.com>
To:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        wuqiang <wuqiang.matt@...edance.com>
Subject: [PATCH bpf-next v5] bpf: Initialize same number of free nodes for each pcpu_freelist

From: Xu Kuohai <xukuohai@...wei.com>

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@...wei.com>
Acked-by: Yonghong Song <yhs@...com>
---
v5: Move "head = per_cpu_ptr ..." out of the i-loop as suggesetd by wuqiang
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 | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
index b6e7f5c5b9ab..de336e3dffee 100644
--- a/kernel/bpf/percpu_freelist.c
+++ b/kernel/bpf/percpu_freelist.c
@@ -100,22 +100,19 @@ 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_idx = 0, cpu, 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();
 
 	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++) {
+			/* No locking required as this is not visible yet. */
+			pcpu_freelist_push_node(head, buf);
+			buf += elem_size;
+		}
 	}
 }
 
-- 
2.30.2