Re: [PATCH] bcachefs: Remove the handling of bch2_trans_iter_exit() in __bch2_bkey_get_iter()

[Hongbo Li]

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@xxxxxxxxxx>
> > > > >
> > > > > - 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,
> > >
> > > 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>