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

[Youling Tang]

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,

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>