linux-kernel - Re: [External] Re: Re: [PATCH v2] bpf: Simplify code by using for_each_cpu

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Date:   Fri, 09 Sep 2022 09:59:07 +0100
From:   Punit Agrawal <punit.agrawal@...edance.com>
To:     Song Liu <song@...nel.org>
Cc:     Punit Agrawal <punit.agrawal@...edance.com>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        zhoufeng.zf@...edance.com, Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [External] Re: Re: [PATCH v2] bpf: Simplify code by using
 for_each_cpu_wrap()

Song Liu <song@...nel.org> writes:

> On Thu, Sep 8, 2022 at 3:45 AM Punit Agrawal
> <punit.agrawal@...edance.com> wrote:
>>
>> Hi Song,
>>
>> Thanks for taking a look.
>>
>> Song Liu <song@...nel.org> writes:
>>
>> > On Wed, Sep 7, 2022 at 8:58 AM Punit Agrawal
>> > <punit.agrawal@...edance.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@...edance.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.

That's true, indeed. While putting the patch together I wondering about
the need for a simpler / optimized version of for_each_cpu_wrap().

> OTOH, the nesting is not too deep (two loops then one if), so I guess
> current version is fine.
>
> Acked-by: Song Liu <song@...nel.org>
>

Thanks!

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