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From:   Song Liu <song@kernel.org>
Date:   Thu, 8 Sep 2022 13:21:04 -0700
Message-ID: <CAPhsuW6fUuc6E7_EoY1h-cikDAT6AuLYCwb89JnaTeOcdrsNFw@mail.gmail.com>
Subject: Re: Re: [PATCH v2] bpf: Simplify code by using for_each_cpu_wrap()
To:     Punit Agrawal <punit.agrawal@bytedance.com>
Cc:     Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        zhoufeng.zf@bytedance.com, Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Jiri Olsa <jolsa@kernel.org>
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On Thu, Sep 8, 2022 at 3:45 AM Punit Agrawal
<punit.agrawal@bytedance.com> wrote:
>
> Hi Song,
>
> Thanks for taking a look.
>
> Song Liu <song@kernel.org> writes:
>
> > On Wed, Sep 7, 2022 at 8:58 AM Punit Agrawal
> > <punit.agrawal@bytedance.com> wrote:
> >>
> >> In the percpu freelist code, it is a common pattern to iterate over
> >> the possible CPUs mask starting with the current CPU. The pattern is
> >> implemented using a hand rolled while loop with the loop variable
> >> increment being open-coded.
> >>
> >> Simplify the code by using for_each_cpu_wrap() helper to iterate over
> >> the possible cpus starting with the current CPU. As a result, some of
> >> the special-casing in the loop also gets simplified.
> >>
> >> No functional change intended.
> >>
> >> Signed-off-by: Punit Agrawal <punit.agrawal@bytedance.com>
> >> ---
> >> v1 -> v2:
> >> * Fixed the incorrect transformation changing semantics of __pcpu_freelist_push_nmi()
> >>
> >> Previous version -
> >> v1: https://lore.kernel.org/all/20220817130807.68279-1-punit.agrawal@bytedance.com/
> >>
> >>  kernel/bpf/percpu_freelist.c | 48 ++++++++++++------------------------
> >>  1 file changed, 16 insertions(+), 32 deletions(-)
> >>
> >> diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
> >> index 00b874c8e889..b6e7f5c5b9ab 100644
> >> --- a/kernel/bpf/percpu_freelist.c
> >> +++ b/kernel/bpf/percpu_freelist.c
> >> @@ -58,23 +58,21 @@ static inline void ___pcpu_freelist_push_nmi(struct pcpu_freelist *s,
> >>  {
> >>         int cpu, orig_cpu;
> >>
> >> -       orig_cpu = cpu = raw_smp_processor_id();
> >> +       orig_cpu = raw_smp_processor_id();
> >>         while (1) {
> >> -               struct pcpu_freelist_head *head;
> >> +               for_each_cpu_wrap(cpu, cpu_possible_mask, orig_cpu) {
> >> +                       struct pcpu_freelist_head *head;
> >>
> >> -               head = per_cpu_ptr(s->freelist, cpu);
> >> -               if (raw_spin_trylock(&head->lock)) {
> >> -                       pcpu_freelist_push_node(head, node);
> >> -                       raw_spin_unlock(&head->lock);
> >> -                       return;
> >> +                       head = per_cpu_ptr(s->freelist, cpu);
> >> +                       if (raw_spin_trylock(&head->lock)) {
> >> +                               pcpu_freelist_push_node(head, node);
> >> +                               raw_spin_unlock(&head->lock);
> >> +                               return;
> >> +                       }
> >>                 }
> >> -               cpu = cpumask_next(cpu, cpu_possible_mask);
> >> -               if (cpu >= nr_cpu_ids)
> >> -                       cpu = 0;
> >
> > I personally don't like nested loops here. Maybe we can keep
> > the original while loop and use cpumask_next_wrap()?
>
> Out of curiosity, is there a reason to avoid nesting here? The nested
> loop avoids the "cpu == orig_cpu" unnecessary check every iteration.

for_each_cpu_wrap is a more complex loop, so we are using some
checks either way.

OTOH, the nesting is not too deep (two loops then one if), so I guess
current version is fine.

Acked-by: Song Liu <song@kernel.org>


>
> As suggested, it's possible to use cpumask_next_wrap() like below -
>
> diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
> index 00b874c8e889..19e8eab70c40 100644
> --- a/kernel/bpf/percpu_freelist.c
> +++ b/kernel/bpf/percpu_freelist.c
> @@ -68,9 +68,7 @@ static inline void ___pcpu_freelist_push_nmi(struct pcpu_freelist *s,
>                         raw_spin_unlock(&head->lock);
>                         return;
>                 }
> -               cpu = cpumask_next(cpu, cpu_possible_mask);
> -               if (cpu >= nr_cpu_ids)
> -                       cpu = 0;
> +               cpu = cpumask_next_wrap(cpu, cpu_possible_mask, orig_cpu, false);
>
>                 /* cannot lock any per cpu lock, try extralist */
>                 if (cpu == orig_cpu &&
>
>
> I can send an updated patch if this is preferred.
>
> Thanks,
> Punit