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Subject[RFC] Perfomance varies according to sysctl_sched_migration_cost
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Hi all,

I noticed that some benchmark performance varies after tunning the sysctl_sched_migration_cost
through /sys/kernel/debug/sched/migration_cost_ns on arm64. The default value is 500000, and
I tried 10000, 100000, 1000000. Below are some results from mmtests, based on 5.14-release.

On Kunpeng920 (128cores, 4numa, 2socket):

pgbench (config-db-pgbench-timed-ro-medium)
mig-cost-500000 mig-cost-100000 mig-cost-10000 mig-cost-1000000
Hmean 1 9558.99 ( 0.00%) 9735.31 * 1.84%* 9410.84 * -1.55%* 9602.47 * 0.45%*
Hmean 8 17615.90 ( 0.00%) 17439.78 * -1.00%* 18056.44 * 2.50%* 19222.18 * 9.12%*
Hmean 12 25228.38 ( 0.00%) 25592.69 * 1.44%* 26739.06 * 5.99%* 27575.48 * 9.30%*
Hmean 24 46623.27 ( 0.00%) 48853.30 * 4.78%* 47386.02 * 1.64%* 48542.94 * 4.12%*
Hmean 32 60578.78 ( 0.00%) 62116.81 * 2.54%* 59961.36 * -1.02%* 58681.07 * -3.13%*
Hmean 48 68159.12 ( 0.00%) 67867.90 ( -0.43%) 65631.79 * -3.71%* 66487.16 * -2.45%*
Hmean 80 66894.87 ( 0.00%) 73440.92 * 9.79%* 68751.63 * 2.78%* 67326.70 ( 0.65%)
Hmean 112 68582.27 ( 0.00%) 65339.90 * -4.73%* 68454.99 ( -0.19%) 67211.66 * -2.00%*
Hmean 144 76290.98 ( 0.00%) 70455.65 * -7.65%* 64851.23 * -14.99%* 64940.61 * -14.88%*
Hmean 172 63245.68 ( 0.00%) 68790.24 * 8.77%* 66246.46 * 4.74%* 69536.96 * 9.95%*
Hmean 204 61793.47 ( 0.00%) 63711.62 * 3.10%* 66055.64 * 6.90%* 58023.20 * -6.10%*
Hmean 236 61486.75 ( 0.00%) 68404.44 * 11.25%* 70499.70 * 14.66%* 58285.67 * -5.21%*
Hmean 256 57476.13 ( 0.00%) 65645.83 * 14.21%* 69437.05 * 20.81%* 60518.05 * 5.29%*

tbench (config-network-tbench)
mig-cost-500000 mig-cost-100000 mig-cost-10000 mig-cost-1000000
Hmean 1 333.12 ( 0.00%) 332.93 ( -0.06%) 335.34 * 0.67%* 334.36 * 0.37%*
Hmean 2 665.88 ( 0.00%) 667.19 * 0.20%* 666.47 * 0.09%* 667.02 * 0.17%*
Hmean 4 1324.10 ( 0.00%) 1312.23 * -0.90%* 1313.07 * -0.83%* 1315.13 * -0.68%*
Hmean 8 2618.85 ( 0.00%) 2602.00 * -0.64%* 2577.49 * -1.58%* 2600.48 * -0.70%*
Hmean 16 5100.74 ( 0.00%) 5068.80 * -0.63%* 5041.34 * -1.16%* 5069.78 * -0.61%*
Hmean 32 8157.22 ( 0.00%) 8163.50 ( 0.08%) 7936.25 * -2.71%* 8329.18 * 2.11%*
Hmean 64 4824.56 ( 0.00%) 4890.81 * 1.37%* 5319.97 * 10.27%* 4830.68 * 0.13%*
Hmean 128 4635.17 ( 0.00%) 6810.90 * 46.94%* 5304.36 * 14.44%* 4516.06 * -2.57%*
Hmean 256 8816.62 ( 0.00%) 8851.28 * 0.39%* 8448.76 * -4.17%* 6840.12 * -22.42%*
Hmean 512 7825.56 ( 0.00%) 8538.04 * 9.10%* 8002.77 * 2.26%* 7946.54 * 1.55%*

Also on Raspberrypi 4B:

pgbench (config-db-pgbench-timed-ro-medium)
mig-cost-500000 mig-cost-100000
Hmean 1 1651.41 ( 0.00%) 3444.27 * 108.56%*
Hmean 4 4015.83 ( 0.00%) 6883.21 * 71.40%*
Hmean 7 4161.45 ( 0.00%) 6646.18 * 59.71%*
Hmean 8 4277.28 ( 0.00%) 6764.60 * 58.15%*

For tbench on Raspberrypi 4B and both pgbench and tbench on x86, tuning sysctl_sched_migration_cost
doesn't have such huge difference and will have some degradations (max -8% on x86 for pgbench) in some cases.

The sysctl_sched_migration_cost will affects the frequency of load balance. It will affect
directly in task_hot() and newidle_balance() to decide whether we can do a migration or load
balance. And affects other parameters like rq->avg_idle, rq->max_idle_balance_cost and
sd->max_newidle_lb_cost to indirectly affect the load balance process. These parameters record
the load_balance() cost and will be limited up to sysctl_sched_migration_cost, so I measure
the average cost of load_balance() on Kunpeng920 with bcc tools(./funclantency load_balance -d 10):

system status idle 50%load 100%load
avg cost 3160ns 4790ns 7563ns

The average cost of load balance seems quite smaller than the default sysctl_sched_migration_cost
which is 500000(500ms).

So I have some RFC questions:
1. how is the default 500000 (500ms) migration cost is measured or caculated?
The value has never changed in the past decade. I dig into the git commits and find it was introduced
in da84d9617672 ("sched: reintroduce cache-hot affinity"). But it didn't explain how did this value come.
2. The ABI now has been removed from sysctl and moved to debugfs. As tuning this can improve the performance
of some workloads on some platforms, maybe it's better to make it a formal sysctl again with docs?

I'll be appreciated for any comments and replies!

Thanks,
Yicong


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