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%

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

Co-developed-by: Zhang Qiao <zhangqiao22@huawei.com>
Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
Signed-off-by: Li Zetao <lizetao1@huawei.com>
---
v1 -> v2: Fix incorrect SoB chain
v1: https://lore.kernel.org/all/20240723073607.568819-1-lizetao1@huawei.com/

 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

On Wed, Jul 31, 2024 at 05:24:00PM GMT, 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%
> 
> 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
> 
> Co-developed-by: Zhang Qiao <zhangqiao22@huawei.com>
> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
> Signed-off-by: Li Zetao <lizetao1@huawei.com>

I ran the same tests on lmbench on Power9 (ppc64le)

Size Comparison

        text	   data	    bss	    dec	    hex	filename
before 111311	  14943	    440	 126694	  1eee6	kernel/sched/fair.o.before
after  111679	  14903	    440	 127022	  1f02e	kernel/sched/fair.o.after

Test comparison
                    before      after     % diff
fork+exit	          698.375	    688	      -1.51%
fork+execve	        2748	      2730.5	  -0.64%
fork+/bin/sh -c	    4298	      4293.5	  -0.10%

Tested-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>
Reviewed-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>

Thanks,
Mukesh