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

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Date:   Wed, 17 Aug 2022 17:57:29 +0100
From:   Punit Agrawal <punit.agrawal@...edance.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Punit Agrawal <punit.agrawal@...edance.com>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: Re: [PATCH] bpf: Simplify code by using for_each_cpu_wrap()

Hi Alexei,

Alexei Starovoitov <alexei.starovoitov@...il.com> writes:

> On Wed, Aug 17, 2022 at 6:08 AM Punit Agrawal
> <punit.agrawal@...edance.com> wrote:
>>
>> No functional change intended.
>
> ?
>
>> -       orig_cpu = cpu = raw_smp_processor_id();
>> -       while (1) {
>> +       for_each_cpu_wrap(cpu, cpu_possible_mask, raw_smp_processor_id()) {
>>                 struct pcpu_freelist_head *head;
>>
>>                 head = per_cpu_ptr(s->freelist, cpu);
>> @@ -68,15 +67,10 @@ 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;
>> -
>> -               /* cannot lock any per cpu lock, try extralist */
>> -               if (cpu == orig_cpu &&
>> -                   pcpu_freelist_try_push_extra(s, node))
>> -                       return;
>>         }
>> +
>> +       /* cannot lock any per cpu lock, try extralist */
>> +       pcpu_freelist_try_push_extra(s, node);
>
> This is obviously not equivalent!

Thanks for taking a look. You're right - I missed the fact that it's an
infinite loop until the node gets pushed to one of the lists.

I'll send an update with that fixed up.