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

[Xu Kuohai]

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++;
> >       }
> >   }
>
> .