linux-kernel - Re: [PATCH -next] sched/fair: inline cpu_util_without and cpu

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Message-ID: <20240724105338.GA13387@noisy.programming.kicks-ass.net>
Date: Wed, 24 Jul 2024 12:53:38 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Li Zetao <lizetao1@...wei.com>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, zhangqiao22@...wei.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] sched/fair: inline cpu_util_without and cpu_util
 to improve performance

On Tue, Jul 23, 2024 at 03:36:07PM +0800, Li Zetao wrote:
> The commit 3eb6d6ececca ("sched/fair: Refactor CPU utilization functions")
> refactored cpu_util_without and cpu_util functions. Since the size of
> cpu_util function has increased, the inline cpu_util is dropped. This had
> a negative impact on performance, in the scenario of updating
> sched_group's statistics, cpu_util_without and cpu_util functions are on
> the hotspot path.
> 
> Inlining cpu_util_without and cpu_util functions have been shown to
> significantly improve performance in lmbench as follow:
> 
>   Machine: Intel(R) Xeon(R) Gold 6248 CPU @ 2.50GHz
>                      before          after          diff
>   fork+exit          317.0625        303.6667       -4.22%
>   fork+execve        1482.5000       1407.0000      -5.09%
>   fork+/bin/sh       2096.0000       2020.3333      -3.61%

That's quite significant. Did you look at what exactly was causing this?

> This patch introduces inlining to cpu_util_without and cpu_util functions.
> While this increases the size of kernel/sched/fair.o, the performance
> gains in critical workloads make this an acceptable trade-off.
> 
> Size comparison before and after patch:
>      text	   data	    bss	    dec	    hex	filename
>    0x1264a	 0x1506	   0xb0	  80896	  13c00	kernel/sched/fair.o.before
>    0x12672	 0x14fe	   0xb0	  80928	  13c20	kernel/sched/fair.o.after
> 
> Signed-off-by: Zhang Qiao <zhangqiao22@...wei.com>
> Signed-off-by: Li Zetao <lizetao1@...wei.com>

This SoB chain is not valid. Please review the documentation we have
on this.

> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5904405ffc59..677b78fa65b6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7706,7 +7706,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   *
>   * Return: (Boosted) (estimated) utilization for the specified CPU.
>   */
> -static unsigned long
> +static __always_inline unsigned long
>  cpu_util(int cpu, struct task_struct *p, int dst_cpu, int boost)
>  {
>  	struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
> @@ -7794,7 +7794,7 @@ unsigned long cpu_util_cfs_boost(int cpu)
>   * utilization of the specified task, whenever the task is currently
>   * contributing to the CPU utilization.
>   */
> -static unsigned long cpu_util_without(int cpu, struct task_struct *p)
> +static __always_inline unsigned long cpu_util_without(int cpu, struct task_struct *p)
>  {
>  	/* Task has no contribution or is new */
>  	if (cpu != task_cpu(p) || !READ_ONCE(p->se.avg.last_update_time))
> -- 
> 2.34.1
>