Optimizing kernel compilation / alignments for network performance

[Rafał Miłecki]

On 27.04.2022 19:31, Rafał Miłecki wrote:
> On 27.04.2022 14:56, Alexander Lobakin wrote:
>> From: Rafał Miłecki <zajec5 at gmail.com>
>> Date: Wed, 27 Apr 2022 14:04:54 +0200
>>
>>> I noticed years ago that kernel changes touching code - that I don't use
>>> at all - can affect network performance for me.
>>>
>>> I work with home routers based on Broadcom Northstar platform. Those
>>> are SoCs with not-so-powerful 2 x ARM Cortex-A9 CPU cores. Main task of
>>> those devices is NAT masquerade and that is what I test with iperf
>>> running on two x86 machines.
>>>
>>> ***
>>>
>>> Example of such unused code change:
>>> ce5013ff3bec ("mtd: spi-nor: Add support for XM25QH64A and XM25QH128A").
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce5013ff3bec05cf2a8a05c75fcd520d9914d92b
>>> It lowered my NAT speed from 381 Mb/s to 367 Mb/s (-3,5%).
>>>
>>> I first reported that issue it in the e-mail thread:
>>> ARM router NAT performance affected by random/unrelated commits
>>> https://lkml.org/lkml/2019/5/21/349
>>> https://www.spinics.net/lists/linux-block/msg40624.html
>>>
>>> Back then it was commit 5b0890a97204 ("flow_dissector: Parse batman-adv
>>> unicast headers")
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9316a9ed6895c4ad2f0cde171d486f80c55d8283
>>> that increased my NAT speed from 741 Mb/s to 773 Mb/s (+4,3%).
>>>
>>> ***
>>>
>>> It appears Northstar CPUs have little cache size and so any change in
>>> location of kernel symbols can affect NAT performance. That explains why
>>> changing unrelated code affects anything & it has been partially proven
>>> aligning some of cache-v7.S code.
>>>
>>> My question is: is there a way to find out & force an optimal symbols
>>> locations?
>>
>> Take a look at CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B[0]. I've been
>> fighting with the same issue on some Realtek MIPS boards: random
>> code changes in random kernel core parts were affecting NAT /
>> network performance. This option resolved this I'd say, for the cost
>> of slightly increased vmlinux size (almost no change in vmlinuz
>> size).
>> The only thing is that it was recently restricted to a set of
>> architectures and MIPS and ARM32 are not included now lol. So it's
>> either a matter of expanding the list (since it was restricted only
>> because `-falign-functions=` is not supported on some architectures)
>> or you can just do:
>>
>> make KCFLAGS=-falign-functions=64 # replace 64 with your I-cache size
>>
>> The actual alignment is something to play with, I stopped on the
>> cacheline size, 32 in my case.
>> Also, this does not provide any guarantees that you won't suffer
>> from random data cacheline changes. There were some initiatives to
>> introduce debug alignment of data as well, but since function are
>> often bigger than 32, while variables are usually much smaller, it
>> was increasing the vmlinux size by a ton (imagine each u32 variable
>> occupying 32-64 bytes instead of 4). But the chance of catching this
>> is much lower than to suffer from I-cache function misplacement.
> 
> Thank you Alexander, this appears to be helpful! I decided to ignore
> CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B for now and just adjust CFLAGS
> manually.
> 
> 
> 1. Without ce5013ff3bec and with -falign-functions=32
> 387 Mb/s
> 
> 2. Without ce5013ff3bec and with -falign-functions=64
> 377 Mb/s
> 
> 3. With ce5013ff3bec and with -falign-functions=32
> 384 Mb/s
> 
> 4. With ce5013ff3bec and with -falign-functions=64
> 377 Mb/s
> 
> 
> So it seems that:
> 1. -falign-functions=32 = pretty stable high speed
> 2. -falign-functions=64 = very stable slightly lower speed
> 
> 
> I'm going to perform tests on more commits but if it stays so reliable
> as above that will be a huge success for me.

So sadly that doesn't work all the time. Or maybe just works randomly.

I tried multiple commits with both: -falign-functions=32 and
-falign-functions=64 . I still get speed variations. About 30 Mb/s in
total. From commit to commit it's usually about 3% but skipping few can
result in up to 30 Mb/s (almost 10%).

Similarly to code changes performance also gets affected by enabling /
disabling kernel config options. I noticed that enabling
CONFIG_CRYPTO_PCRYPT may decrease *or* increase speed depending on
-falign-functions (and depending on kernel commit surely too).

┌──────────────────────┬───────────┬──────────┬───────┐
│                      │ no PCRYPT │ PCRYPT=y │ diff  │
├──────────────────────┼───────────┼──────────┼───────┤
│ No -falign-functions │ 363 Mb/s  │ 370 Mb/s │ +2%   │
│ -falign-functions=32 │ 364 Mb/s  │ 370 Mb/s │ +1,7% │
│ -falign-functions=64 │ 372 Mb/s  │ 365 Mb/s │ -2%   │
└──────────────────────┴───────────┴──────────┴───────┘

So I still don't have a reliable way of testing kernel changes for speed
regressions :(