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

[Xu Kuohai]

On 11/12/2022 4:12 AM, Andrii Nakryiko wrote:
> On Thu, Nov 10, 2022 at 11:00 PM Xu Kuohai <xukuohai@xxxxxxxxxx> wrote:
> > 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
>
> I did that while applying. Also added
>
> Fixes: e19494edab82 ("bpf: introduce percpu_freelist")
>
> Please don't forget to add Fixes tag for future patches.

OK, thanks for the kind reminder

> Applied to bpf tree.
>
> > > 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++;
> > > >        }
> > > >    }
> > >
> > > .
> .