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Subject: Re: [PATCH v2 bpf] bpf: Do not grab the bucket spinlock by default on
 htab batch ops
To:     Brian Vazquez <brianvv@google.com>,
        Brian Vazquez <brianvv.kernel@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "David S . Miller" <davem@davemloft.net>
CC:     <linux-kernel@vger.kernel.org>, <netdev@vger.kernel.org>,
        <bpf@vger.kernel.org>
References: <20200218172552.215077-1-brianvv@google.com>
From:   Yonghong Song <yhs@fb.com>
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Date:   Tue, 18 Feb 2020 13:23:15 -0800
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On 2/18/20 9:25 AM, Brian Vazquez wrote:
> Grabbing the spinlock for every bucket even if it's empty, was causing
> significant perfomance cost when traversing htab maps that have only a
> few entries. This patch addresses the issue by checking first the
> bucket_cnt, if the bucket has some entries then we go and grab the
> spinlock and proceed with the batching.
> 
> Tested with a htab of size 50K and different value of populated entries.
> 
> Before:
>    Benchmark             Time(ns)        CPU(ns)
>    ---------------------------------------------
>    BM_DumpHashMap/1       2759655        2752033
>    BM_DumpHashMap/10      2933722        2930825
>    BM_DumpHashMap/200     3171680        3170265
>    BM_DumpHashMap/500     3639607        3635511
>    BM_DumpHashMap/1000    4369008        4364981
>    BM_DumpHashMap/5k     11171919       11134028
>    BM_DumpHashMap/20k    69150080       69033496
>    BM_DumpHashMap/39k   190501036      190226162
> 
> After:
>    Benchmark             Time(ns)        CPU(ns)
>    ---------------------------------------------
>    BM_DumpHashMap/1        202707         200109
>    BM_DumpHashMap/10       213441         210569
>    BM_DumpHashMap/200      478641         472350
>    BM_DumpHashMap/500      980061         967102
>    BM_DumpHashMap/1000    1863835        1839575
>    BM_DumpHashMap/5k      8961836        8902540
>    BM_DumpHashMap/20k    69761497       69322756
>    BM_DumpHashMap/39k   187437830      186551111
> 
> Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
> Cc: Yonghong Song <yhs@fb.com>
> Signed-off-by: Brian Vazquez <brianvv@google.com>

Thanks for the fix.
Acked-by: Yonghong Song <yhs@fb.com>

> ---
> v1 -> v2: Skip hlist_nulls_for_each_entry_safe if lock is not held
> 
>   kernel/bpf/hashtab.c | 22 +++++++++++++++++++++-
>   1 file changed, 21 insertions(+), 1 deletion(-)