Re: [PATCH v2] sched/fair: bump sd->max_newidle_lb_cost when newidle balance fails

["Chen, Yu C" <yu.c.chen@intel.com>]

On 7/15/2025 3:08 PM, kernel test robot wrote:
> 
> 
> Hello,
> 
> kernel test robot noticed a 22.9% regression of unixbench.throughput on:
> 
> 
> commit: ac34cb39e8aea9915ec2f4e08c979eb2ed1d7561 ("[PATCH v2] sched/fair: bump sd->max_newidle_lb_cost when newidle balance fails")
> url: https://github.com/intel-lab-lkp/linux/commits/Chris-Mason/sched-fair-bump-sd-max_newidle_lb_cost-when-newidle-balance-fails/20250626-224805
> base: https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git 5bc34be478d09c4d16009e665e020ad0fcd0deea
> patch link: https://lore.kernel.org/all/20250626144017.1510594-2-clm@fb.com/
> patch subject: [PATCH v2] sched/fair: bump sd->max_newidle_lb_cost when newidle balance fails
> 
> testcase: unixbench
> config: x86_64-rhel-9.4
> compiler: gcc-12
> test machine: 64 threads 2 sockets Intel(R) Xeon(R) Gold 6346 CPU @ 3.10GHz (Ice Lake) with 256G memory
> parameters:
> 
> 	runtime: 300s
> 	nr_task: 100%
> 	test: shell1
> 	cpufreq_governor: performance
> 
> 
...

> 
> commit:
>    5bc34be478 ("sched/core: Reorganize cgroup bandwidth control interface file writes")
>    ac34cb39e8 ("sched/fair: bump sd->max_newidle_lb_cost when newidle balance fails")
> 
> 5bc34be478d09c4d ac34cb39e8aea9915ec2f4e08c9
> ---------------- ---------------------------
>           %stddev     %change         %stddev
>               \          |                \
...

>       40.37           +16.9       57.24        mpstat.cpu.all.idle%

This commit inhibits the newidle balance. It seems that some workloads
do not like newlyidle balance, like schbench, which is short duration
task. While other workloads want the newidle balance to pull at its best
effort, like unixbench shell test case.
Just wonder if we can check the sched domain's average utilization to
decide how hard we should trigger the newly idle balance, or can we check
the overutilized flag to decide whether we should launch the
new idle balance, something I was thinking of:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9e24038fa000..6c7420ed484e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13759,7 +13759,8 @@ static int sched_balance_newidle(struct rq 
*this_rq, struct rq_flags *rf)
         sd = rcu_dereference_check_sched_domain(this_rq->sd);

         if (!get_rd_overloaded(this_rq->rd) ||
-           (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
+           (sd && this_rq->avg_idle < sd->max_newidle_lb_cost &&
+            !READ_ONCE(this_rq->rd->overutilized))) {

                 if (sd)
                         update_next_balance(sd, &next_balance);


thanks,
Chenyu