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

[Feng Zhou]

在 2022/6/1 下午7:35, Alexei Starovoitov 写道:
> On Wed, Jun 1, 2022 at 1:11 PM Feng Zhou <zhoufeng.zf@xxxxxxxxxxxxx> wrote:
> > 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@xxxxxxxxxxxxx> 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.
> maybe. pls benchmark it.
> imo wasting a bool for the corner case is not a good trade off.

Yes, I will do and post the results as soon as possible, Thanks.