From: Daniel Borkmann <daniel@iogearbox.net>
To: Yonghong Song <yhs@fb.com>, Brian Vazquez <brianvv@google.com>,
Brian Vazquez <brianvv.kernel@gmail.com>,
Alexei Starovoitov <ast@kernel.org>,
"David S . Miller" <davem@davemloft.net>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
bpf@vger.kernel.org
Subject: Re: [PATCH bpf] bpf: Do not grab the bucket spinlock by default on htab batch ops
Date: Tue, 18 Feb 2020 16:56:24 +0100 [thread overview]
Message-ID: <63fa17bf-a109-65c1-6cc5-581dd84fc93b@iogearbox.net> (raw)
In-Reply-To: <8ac06749-491f-9a77-3899-641b4f40afe2@fb.com>
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@fb.com>
>> Signed-off-by: Brian Vazquez <brianvv@google.com>
>
> Acked-by: Yonghong Song <yhs@fb.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?
next prev parent reply other threads:[~2020-02-18 15:56 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-14 22:43 [PATCH bpf] bpf: Do not grab the bucket spinlock by default on htab batch ops Brian Vazquez
2020-02-18 15:43 ` Yonghong Song
2020-02-18 15:56 ` Daniel Borkmann [this message]
2020-02-18 16:34 ` Yonghong Song
2020-02-18 17:17 ` Brian Vazquez
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