netdev - Re: [RFC/RFT v2 0/3] Introduce GRO support to cpumap codebase

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Message-ID: <3f6e4935-a04c-44fc-8048-7645ae40b921@kernel.org>
Date: Tue, 26 Nov 2024 18:12:27 +0100
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
 Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Daniel Xu <dxu@...uu.xyz>, Jakub Kicinski <kuba@...nel.org>,
 Lorenzo Bianconi <lorenzo@...nel.org>,
 "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Martin KaFai Lau <martin.lau@...ux.dev>, David Miller <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org
Subject: Re: [RFC/RFT v2 0/3] Introduce GRO support to cpumap codebase




On 26/11/2024 18.02, Lorenzo Bianconi wrote:
>> From: Daniel Xu <dxu@...uu.xyz>
>> Date: Mon, 25 Nov 2024 16:56:49 -0600
>>
>>>
>>>
>>> On Mon, Nov 25, 2024, at 9:12 AM, Alexander Lobakin wrote:
>>>> From: Daniel Xu <dxu@...uu.xyz>
>>>> Date: Fri, 22 Nov 2024 17:10:06 -0700
>>>>
>>>>> Hi Olek,
>>>>>
>>>>> Here are the results.
>>>>>
>>>>> On Wed, Nov 13, 2024 at 03:39:13PM GMT, Daniel Xu wrote:
>>>>>>
>>>>>>
>>>>>> On Tue, Nov 12, 2024, at 9:43 AM, Alexander Lobakin wrote:
>>>>
>>>> [...]
>>>>
>>>>> Baseline (again)
>>>>>
>>>>> 	Transactions	Latency P50 (s)	Latency P90 (s)	Latency P99 (s)			Throughput (Mbit/s)
>>>>> Run 1	3169917	        0.00007295	0.00007871	0.00009343		Run 1	21749.43
>>>>> Run 2	3228290	        0.00007103	0.00007679	0.00009215		Run 2	21897.17
>>>>> Run 3	3226746	        0.00007231	0.00007871	0.00009087		Run 3	21906.82
>>>>> Run 4	3191258	        0.00007231	0.00007743	0.00009087		Run 4	21155.15
>>>>> Run 5	3235653	        0.00007231	0.00007743	0.00008703		Run 5	21397.06
>>>>> Average	3210372.8	0.000072182	0.000077814	0.00009087		Average	21621.126
>>>>>
>>>>> cpumap v2 Olek
>>>>>
>>>>> 	Transactions	Latency P50 (s)	Latency P90 (s)	Latency P99 (s)			Throughput (Mbit/s)
>>>>> Run 1	3253651	        0.00007167	0.00007807	0.00009343		Run 1	13497.57
>>>>> Run 2	3221492	        0.00007231	0.00007743	0.00009087		Run 2	12115.53
>>>>> Run 3	3296453	        0.00007039	0.00007807	0.00009087		Run 3	12323.38
>>>>> Run 4	3254460	        0.00007167	0.00007807	0.00009087		Run 4	12901.88
>>>>> Run 5	3173327	        0.00007295	0.00007871	0.00009215		Run 5	12593.22
>>>>> Average	3239876.6	0.000071798	0.00007807	0.000091638		Average	12686.316
>>>>> Delta	0.92%	        -0.53%	        0.33%	        0.85%			        -41.32%
>>>>>
>>>>>
>>>>> It's very interesting that we see -40% tput w/ the patches. I went back
>>>>
>>>> Oh no, I messed up something =\
>>>>
>>>> Could you please also test not the whole series, but patches 1-3 (up to
>>>> "bpf:cpumap: switch to GRO...") and 1-4 (up to "bpf: cpumap: reuse skb
>>>> array...")? Would be great to see whether this implementation works
>>>> worse right from the start or I just broke something later on.
>>>
>>> Patches 1-3 reproduces the -40% tput numbers.
>>
>> Ok, thanks! Seems like using the hybrid approach (GRO, but on top of
>> cpumap's kthreads instead of NAPI) really performs worse than switching
>> cpumap to NAPI.
>>
>>>
>>> With patches 1-4 the numbers get slightly worse (~1gbps lower) but it was noisy.
>>
>> Interesting, I was sure patch 4 optimizes stuff... Maybe I'll give up on it.
>>
>>>
>>> tcp_rr results were unaffected.
>>
>> @ Jakub,
>>
>> Looks like I can't just use GRO without Lorenzo's conversion to NAPI, at
>> least for now =\ I took a look on the backlog NAPI and it could be used,
>> although we'd need a pointer in the backlog to the corresponding cpumap
>> + also some synchronization point to make sure backlog NAPI won't access
>> already destroyed cpumap.
>>
>> Maybe Lorenzo could take a look...
> 
> it seems to me the only difference would be we will use the shared backlog_napi
> kthreads instead of having a dedicated kthread for each cpumap entry but we still
> need the napi poll logic. I can look into it if you prefer the shared kthread
> approach.

I don't like a shared kthread approach. For my use-case I want to give
the "remote" CPU-map kthreads higher scheduling priority. (As it will be
running a 2nd XDP BPF DDoS program protecting against overload by 
dropping packets).

Thus, I'm not a fan of using the shared backlog_napi.  As I don't want
to give backlog NAPI high priority, in my use-case.

> @Jakub: what do you think?


--Jesper