Re: [External] Re: [PATCH v4 1/2] bpf: avoid grabbing spin_locks of all cpus when no free elems

[Feng Zhou <zhoufeng.zf@bytedance.com>]

在 2022/6/1 下午5:50, Alexei Starovoitov 写道:
> On Wed, Jun 1, 2022 at 10:42 AM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>>   static inline void ___pcpu_freelist_push(struct pcpu_freelist_head *head,
>> @@ -130,14 +134,19 @@ static struct pcpu_freelist_node *___pcpu_freelist_pop(struct pcpu_freelist *s)
>>          orig_cpu = cpu = raw_smp_processor_id();
>>          while (1) {
>>                  head = per_cpu_ptr(s->freelist, cpu);
>> +               if (READ_ONCE(head->is_empty))
>> +                       goto next_cpu;
>>                  raw_spin_lock(&head->lock);
>>                  node = head->first;
>>                  if (node) {
> extra bool is unnecessary.
> just READ_ONCE(head->first)

As for why to add is_empty instead of directly judging head->first, my
understanding is this, head->first is frequently modified during updating
map, which will lead to invalid other cpus's cache, and is_empty is after
freelist having no free elems will be changed, the performance will be 
better.
If I'm thinking wrong, please tell me why.