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

[Xu Kuohai]

On 11/8/2022 12:40 AM, Yonghong Song wrote:
> On 11/7/22 12:50 AM, Xu Kuohai wrote:
> > From: Xu Kuohai <xukuohai@xxxxxxxxxx>
> >
> > pcpu_freelist_populate() initializes nr_elems / num_possible_cpus() + 1
> > free nodes for each CPU except the last initialized CPU, always making
> > the last CPU get fewer free nodes. For example, when nr_elems == 256
>
> ... free nodes for some cpus, and then possibly one cpu with fewer nodes, followed by remaining cpus with 0 nodes.

Will update the commit message to describe it more accurately, thanks.

> > and num_possible_cpus() == 32, if CPU 0 is the current cpu, 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, and then allocates the remaining free nodes by one for each
> > CPU until no free nodes left.
> >
> > Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
>
> LGTM. Did you observe any performance issues?

No. I ran map_perf_test and did not observe any performance issues. I think
it's because the test cases are repeated in loops, so the pcpu_freelists become
stable and balanced after the first few loops.

> Acked-by: Yonghong Song <yhs@xxxxxx>
>
> > ---
> >   kernel/bpf/percpu_freelist.c | 9 ++++++---
> >   1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
> > index b6e7f5c5b9ab..89e84f7381cc 100644
> > --- a/kernel/bpf/percpu_freelist.c
> > +++ b/kernel/bpf/percpu_freelist.c
> > @@ -100,12 +100,15 @@ 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;
> > +    int i, cpu, pcpu_entries, remain_entries;
> > +
> > +    pcpu_entries = nr_elems / num_possible_cpus();
> > +    remain_entries = nr_elems % num_possible_cpus();
> > -    pcpu_entries = nr_elems / num_possible_cpus() + 1;
> >       i = 0;
> >       for_each_possible_cpu(cpu) {
> > +        int j = i + pcpu_entries + (remain_entries-- > 0 ? 1 : 0);
> >   again:
> >           head = per_cpu_ptr(s->freelist, cpu);
> >           /* No locking required as this is not visible yet. */
> > @@ -114,7 +117,7 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
> >           buf += elem_size;
> >           if (i == nr_elems)
> >               break;
> > -        if (i % pcpu_entries)
> > +        if (i < j)
> >               goto again;
> >       }
> >   }