linux-kernel - Re: [PATCH] bcachefs: Remove the handling of bch2_trans_iter_exit() in __bch2_bkey_get

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Message-ID: <3f60f1de-f96c-434b-a4de-355100ce9e79@huawei.com>
Date: Fri, 30 Aug 2024 11:53:11 +0800
From: Hongbo Li <lihongbo22@...wei.com>
To: Youling Tang <youling.tang@...ux.dev>
CC: <linux-bcachefs@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Youling
 Tang <tangyouling@...inos.cn>, Kent Overstreet <kent.overstreet@...ux.dev>
Subject: Re: [PATCH] bcachefs: Remove the handling of bch2_trans_iter_exit()
 in __bch2_bkey_get_iter()



On 2024/8/30 11:48, Hongbo Li wrote:
> 
> 
> On 2024/8/30 9:19, Youling Tang wrote:
>> On 2024/8/23 22:51, Kent Overstreet wrote:
>>
>>> On Fri, Aug 23, 2024 at 02:07:20PM GMT, Youling Tang wrote:
>>>> On 23/08/2024 11:55, Kent Overstreet wrote:
>>>>> On Fri, Aug 23, 2024 at 11:19:55AM GMT, Youling Tang wrote:
>>>>>> From: Youling Tang <tangyouling@...inos.cn>
>>>>>>
>>>>>> - Reduces bkey_err() calls.
>>>>>> - Avoid redundant calls to bch2_trans_iter_exit() in some functions.
>>>>> no, a function that returns an error should clean up after itself
>>>> Yes, functions should self-clean when they fail.
>>>>
>>>> However, there are repeated calls to bch2_trans_iter_exit in
>>>> some functions, take lookup_inode() as an example,
>>>>
This is indeed the case. Personally, I think the bcachefs code is quite 
difficult to understand (such as too many macro, etc..), with many parts 
that could be simplified. I think may be only Kent himself can fully 
hold it.

>>>> When bkey_err(k) returns a non-zero, call bch2_trans_iter_exit()
>>>> once in bch2_bkey_get_iter(). It is then called again in
>>>> lookup_inode() via 'goto err'. (We can correct it by simply changing
>>>> it to 'return ret', but there are many similar cases.)
>>> I'm aware, but I'm not looking to microoptimize at the expense of making
>>> the code more fragile and less clear, especially right now when the
>>> priority is stabilizing and fixing bugs.
>>>
>>> If you were also doing performance testing and could show that it
>>> makes a measurable difference I'd consider it. Did you even look at the
>>> assembly output for any of these functions? CSE might be optimizing away
>>> the redundant calls.
>> I haven't performed the corresponding performance testing. Looking at
>> the assembly code, taking `lookup_inode()` as an example,
>>
> I think his point is that your modification is error-free, but improving 
> readability is not a high priority at the moment unless it actually 
> affects performance. There might currently be a shortage of manpower, 
> with many features to be completed and bugs to be fixed.
>> Before the patch,
>>      142f:       74 96 je     13c7 <lookup_inode+0x117>
>>      1431:       48 8d b5 68 ff ff ff lea    -0x98(%rbp),%rsi
>>      1438:       4c 89 e7 mov    %r12,%rdi
>>      143b:       e8 00 00 00 00 call   1440 <lookup_inode+0x190>
>>      1440:       eb b4 jmp    13f6 <lookup_inode+0x146>
>>      1442:       e8 00 00 00 00 call   1447 <lookup_inode+0x197>
>>      1447:       66 0f 1f 84 00 00 00 nopw   0x0(%rax,%rax,1)
>>      144e:       00 00
>>
>> After,
>>      111f:       74 96 je     10b7 <lookup_inode+0x117>
>>      1121:       eb c3 jmp    10e6 <lookup_inode+0x146>
>>      1123:       e8 00 00 00 00 call   1128 <lookup_inode+0x188>
>>      1128:       0f 1f 84 00 00 00 00 nopl   0x0(%rax,%rax,1)
>>      112f:       00
>>
>> The following three assembly instructions have been reduced,
>>      1431:       48 8d b5 68 ff ff ff lea    -0x98(%rbp),%rsi
>>      1438:       4c 89 e7 mov    %r12,%rdi
>>      143b:       e8 00 00 00 00 call   1440 <lookup_inode+0x190>
>>