linux-kernel - Re: [PATCH] net/smc: Reduce size of smc_wr_tx_tasklet

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Message-ID: <6191739c-93db-4a7d-8e83-3168909315cd@linux.ibm.com>
Date: Tue, 18 Mar 2025 09:43:07 +0100
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: I Hsin Cheng <richard120310@...il.com>, alibuda@...ux.alibaba.com,
        jaka@...ux.ibm.com, mjambigi@...ux.ibm.com, sidraya@...ux.ibm.com,
        tonylu@...ux.alibaba.com, guwen@...ux.alibaba.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        horms@...nel.org, linux-rdma@...r.kernel.org,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
        jserv@...s.ncku.edu.tw, linux-kernel-mentees@...ts.linux.dev
Subject: Re: [PATCH] net/smc: Reduce size of smc_wr_tx_tasklet_fn



On 17.03.25 14:56, Heiko Carstens wrote:
> On Mon, Mar 17, 2025 at 12:22:46PM +0100, Wenjia Zhang wrote:
>>
>>
>> On 15.03.25 07:25, I Hsin Cheng wrote:
>>> The variable "polled" in smc_wr_tx_tasklet_fn is a counter to determine
>>> whether the loop has been executed for the first time. Refactor the type
>>> of "polled" from "int" to "bool" can reduce the size of generated code
>>> size by 12 bytes shown with the test below
>>>
>>> $ ./scripts/bloat-o-meter vmlinux_old vmlinux_new
>>> add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-12 (-12)
>>> Function                                     old     new   delta
>>> smc_wr_tx_tasklet_fn                        1076    1064     -12
>>> Total: Before=24795091, After=24795079, chg -0.00%
>>>
>>> In some configuration, the compiler will complain this function for
>>> exceeding 1024 bytes for function stack, this change can at least reduce
>>> the size by 12 bytes within manner.
>>>
>> The code itself looks good. However, I’m curious about the specific
>> situation where the compiler complained. Also, compared to exceeding the
>> function stack limit by 1024 bytes, I don’t see how saving 12 bytes would
>> bring any significant benefit.
> 
> The patch description doesn't make sense: bloat-a-meter prints the _text
> size_ difference of two kernels, which really has nothing to do with
> potential stack size savings.
> 
> If there are any changes in stack size with this patch is unknown; at least
> if you rely only on the patch description.
> 
> You may want to have a look at scripts/stackusage and scripts/stackdelta.

@Heiko, thank you for pointing it out!

Even if the potential stack size saving of 12 bytes were true, I still 
don’t see how it would benefit our code, let alone justify the incorrect 
argument.