linux-kernel - Re: [PATCH] sched: core: Simplify cpuset_cpumask_can

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Message-ID: <xhsmhr0qhx9z8.mognet@vschneid.remote.csb>
Date:   Mon, 12 Jun 2023 11:15:23 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Li zeming <zeming@...china.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org, Li zeming <zeming@...china.com>
Subject: Re: [PATCH] sched: core: Simplify cpuset_cpumask_can_shrink()

On 19/05/23 04:34, Li zeming wrote:
> Remove useless intermediate variable "ret" and its initialization.
> Directly return dl_cpuset_cpumask_can_shrink() result.
>

FWIW, this sort of intermediate variable can and will be optimized away by
the compiler. It's pretty obvious to see in the objdump (GCC 13.1):

before/core.o: mainline core.o
after/core.o:  patched core.o

$ gdb -batch -ex 'disassemble cpuset_cpumask_can_shrink' before/core.o
Dump of assembler code for function cpuset_cpumask_can_shrink:
   0x000000000000b260 <+0>:	endbr64
   0x000000000000b264 <+4>:	cmpq   $0x0,(%rdi)
   0x000000000000b268 <+8>:	jne    0xb274 <cpuset_cpumask_can_shrink+20>
   0x000000000000b26a <+10>:	mov    $0x1,%eax
   0x000000000000b26f <+15>:	jmpq   0xb274 <cpuset_cpumask_can_shrink+20>
   0x000000000000b274 <+20>:	jmpq   0xb279


$ diff <(gdb -batch -ex 'disassemble cpuset_cpumask_can_shrink' before/core.o) \
       <(gdb -batch -ex 'disassemble cpuset_cpumask_can_shrink' after/core.o)
<nothing>

Generally I think it's good to not be afraid to use intermediate variables
if they improve readability, even if they have only one use (e.g. as
parameter to a function with many arguments / long expressions used as
parameters).

In this case though, the intermediate variable really doesn't do much for
readability :-)

Reviewed-by: Valentin Schneider <vschneid@...hat.com>

> Signed-off-by: Li zeming <zeming@...china.com>
> ---
>  kernel/sched/core.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a66960da3f5c..f3f2ece26291 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9273,14 +9273,10 @@ void __init init_idle(struct task_struct *idle, int cpu)
>  int cpuset_cpumask_can_shrink(const struct cpumask *cur,
>                             const struct cpumask *trial)
>  {
> -	int ret = 1;
> -
>       if (cpumask_empty(cur))
> -		return ret;
> -
> -	ret = dl_cpuset_cpumask_can_shrink(cur, trial);
> +		return 1;
>
> -	return ret;
> +	return dl_cpuset_cpumask_can_shrink(cur, trial);
>  }
>
>  int task_can_attach(struct task_struct *p,
> --
> 2.18.2