netdev - Re: [PATCH] bpf: use count for prealloc hashtab too

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Date:   Fri, 15 Oct 2021 21:58:40 +0200
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: use count for prealloc hashtab too

On Fri, Oct 15, 2021 at 11:04 AM Chengming Zhou
<zhouchengming@...edance.com> wrote:
>
> We only use count for kmalloc hashtab not for prealloc hashtab, because
> __pcpu_freelist_pop() return NULL when no more elem in pcpu freelist.
>
> But the problem is that __pcpu_freelist_pop() will traverse all CPUs and
> spin_lock for all CPUs to find there is no more elem at last.
>
> We encountered bad case on big system with 96 CPUs that alloc_htab_elem()
> would last for 1ms. This patch use count for prealloc hashtab too,
> avoid traverse and spin_lock for all CPUs in this case.
>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>

It's not clear from the commit log what you're solving.
The atomic inc/dec in critical path of prealloc maps hurts performance.
That's why it's not used.