linux-kernel - Re: [PATCH bpf] bpf: Do not grab the bucket spinlock by default on htab batch ops

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Date:   Tue, 18 Feb 2020 16:56:24 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, Brian Vazquez <brianvv@...gle.com>,
        Brian Vazquez <brianvv.kernel@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: Do not grab the bucket spinlock by default on
 htab batch ops

On 2/18/20 4:43 PM, Yonghong Song wrote:
> On 2/14/20 2:43 PM, 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@...com>
>> Signed-off-by: Brian Vazquez <brianvv@...gle.com>
> 
> Acked-by: Yonghong Song <yhs@...com>

I must probably be missing something, but how is this safe? Presume we
traverse in the walk with bucket_cnt = 0. Meanwhile a different CPU added
entries to this bucket since not locked. Same reader on the other CPU with
bucket_cnt = 0 then starts to traverse the second
hlist_nulls_for_each_entry_safe() unlocked e.g. deleting entries?