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SubjectRe: [PATCH v4] sched/fair: Introduce SIS_UTIL to search idle CPU based on sum of util_avg
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On 2022/6/13 0:34, Chen Yu wrote:
> [Problem Statement]
> select_idle_cpu() might spend too much time searching for an idle CPU,
> when the system is overloaded.
>
> The following histogram is the time spent in select_idle_cpu(),
> when running 224 instances of netperf on a system with 112 CPUs
> per LLC domain:
>
> @usecs:
> [0] 533 | |
> [1] 5495 | |
> [2, 4) 12008 | |
> [4, 8) 239252 | |
> [8, 16) 4041924 |@@@@@@@@@@@@@@ |
> [16, 32) 12357398 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
> [32, 64) 14820255 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [64, 128) 13047682 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
> [128, 256) 8235013 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
> [256, 512) 4507667 |@@@@@@@@@@@@@@@ |
> [512, 1K) 2600472 |@@@@@@@@@ |
> [1K, 2K) 927912 |@@@ |
> [2K, 4K) 218720 | |
> [4K, 8K) 98161 | |
> [8K, 16K) 37722 | |
> [16K, 32K) 6715 | |
> [32K, 64K) 477 | |
> [64K, 128K) 7 | |
>
> netperf latency usecs:
> =======
> case load Lat_99th std%
> TCP_RR thread-224 257.39 ( 0.21)
>
> The time spent in select_idle_cpu() is visible to netperf and might have a negative
> impact.
>
> [Symptom analysis]
> The patch [1] from Mel Gorman has been applied to track the efficiency
> of select_idle_sibling. Copy the indicators here:
>
> SIS Search Efficiency(se_eff%):
> A ratio expressed as a percentage of runqueues scanned versus
> idle CPUs found. A 100% efficiency indicates that the target,
> prev or recent CPU of a task was idle at wakeup. The lower the
> efficiency, the more runqueues were scanned before an idle CPU
> was found.
>
> SIS Domain Search Efficiency(dom_eff%):
> Similar, except only for the slower SIS
> patch.
>
> SIS Fast Success Rate(fast_rate%):
> Percentage of SIS that used target, prev or
> recent CPUs.
>
> SIS Success rate(success_rate%):
> Percentage of scans that found an idle CPU.
>
> The test is based on Aubrey's schedtests tool, including netperf, hackbench,
> schbench and tbench.
>
> Test on vanilla kernel:
> schedstat_parse.py -f netperf_vanilla.log
> case load se_eff% dom_eff% fast_rate% success_rate%
> TCP_RR 28 threads 99.978 18.535 99.995 100.000
> TCP_RR 56 threads 99.397 5.671 99.964 100.000
> TCP_RR 84 threads 21.721 6.818 73.632 100.000
> TCP_RR 112 threads 12.500 5.533 59.000 100.000
> TCP_RR 140 threads 8.524 4.535 49.020 100.000
> TCP_RR 168 threads 6.438 3.945 40.309 99.999
> TCP_RR 196 threads 5.397 3.718 32.320 99.982
> TCP_RR 224 threads 4.874 3.661 25.775 99.767
> UDP_RR 28 threads 99.988 17.704 99.997 100.000
> UDP_RR 56 threads 99.528 5.977 99.970 100.000
> UDP_RR 84 threads 24.219 6.992 76.479 100.000
> UDP_RR 112 threads 13.907 5.706 62.538 100.000
> UDP_RR 140 threads 9.408 4.699 52.519 100.000
> UDP_RR 168 threads 7.095 4.077 44.352 100.000
> UDP_RR 196 threads 5.757 3.775 35.764 99.991
> UDP_RR 224 threads 5.124 3.704 28.748 99.860
>
> schedstat_parse.py -f schbench_vanilla.log
> (each group has 28 tasks)
> case load se_eff% dom_eff% fast_rate% success_rate%
> normal 1 mthread 99.152 6.400 99.941 100.000
> normal 2 mthreads 97.844 4.003 99.908 100.000
> normal 3 mthreads 96.395 2.118 99.917 99.998
> normal 4 mthreads 55.288 1.451 98.615 99.804
> normal 5 mthreads 7.004 1.870 45.597 61.036
> normal 6 mthreads 3.354 1.346 20.777 34.230
> normal 7 mthreads 2.183 1.028 11.257 21.055
> normal 8 mthreads 1.653 0.825 7.849 15.549
>
> schedstat_parse.py -f hackbench_vanilla.log
> (each group has 28 tasks)
> case load se_eff% dom_eff% fast_rate% success_rate%
> process-pipe 1 group 99.991 7.692 99.999 100.000
> process-pipe 2 groups 99.934 4.615 99.997 100.000
> process-pipe 3 groups 99.597 3.198 99.987 100.000
> process-pipe 4 groups 98.378 2.464 99.958 100.000
> process-pipe 5 groups 27.474 3.653 89.811 99.800
> process-pipe 6 groups 20.201 4.098 82.763 99.570
> process-pipe 7 groups 16.423 4.156 77.398 99.316
> process-pipe 8 groups 13.165 3.920 72.232 98.828
> process-sockets 1 group 99.977 5.882 99.999 100.000
> process-sockets 2 groups 99.927 5.505 99.996 100.000
> process-sockets 3 groups 99.397 3.250 99.980 100.000
> process-sockets 4 groups 79.680 4.258 98.864 99.998
> process-sockets 5 groups 7.673 2.503 63.659 92.115
> process-sockets 6 groups 4.642 1.584 58.946 88.048
> process-sockets 7 groups 3.493 1.379 49.816 81.164
> process-sockets 8 groups 3.015 1.407 40.845 75.500
> threads-pipe 1 group 99.997 0.000 100.000 100.000
> threads-pipe 2 groups 99.894 2.932 99.997 100.000
> threads-pipe 3 groups 99.611 4.117 99.983 100.000
> threads-pipe 4 groups 97.703 2.624 99.937 100.000
> threads-pipe 5 groups 22.919 3.623 87.150 99.764
> threads-pipe 6 groups 18.016 4.038 80.491 99.557
> threads-pipe 7 groups 14.663 3.991 75.239 99.247
> threads-pipe 8 groups 12.242 3.808 70.651 98.644
> threads-sockets 1 group 99.990 6.667 99.999 100.000
> threads-sockets 2 groups 99.940 5.114 99.997 100.000
> threads-sockets 3 groups 99.469 4.115 99.977 100.000
> threads-sockets 4 groups 87.528 4.038 99.400 100.000
> threads-sockets 5 groups 6.942 2.398 59.244 88.337
> threads-sockets 6 groups 4.359 1.954 49.448 87.860
> threads-sockets 7 groups 2.845 1.345 41.198 77.102
> threads-sockets 8 groups 2.871 1.404 38.512 74.312
>
> schedstat_parse.py -f tbench_vanilla.log
> case load se_eff% dom_eff% fast_rate% success_rate%
> loopback 28 threads 99.976 18.369 99.995 100.000
> loopback 56 threads 99.222 7.799 99.934 100.000
> loopback 84 threads 19.723 6.819 70.215 100.000
> loopback 112 threads 11.283 5.371 55.371 99.999
> loopback 140 threads 0.000 0.000 0.000 0.000
> loopback 168 threads 0.000 0.000 0.000 0.000
> loopback 196 threads 0.000 0.000 0.000 0.000
> loopback 224 threads 0.000 0.000 0.000 0.000
>
> According to the test above, if the system becomes busy, the
> SIS Search Efficiency(se_eff%) drops significantly. Although some
> benchmarks would finally find an idle CPU(success_rate% = 100%), it is
> doubtful whether it is worth it to search the whole LLC domain.
>
> [Proposal]
> It would be ideal to have a crystal ball to answer this question:
> How many CPUs must a wakeup path walk down, before it can find an idle
> CPU? Many potential metrics could be used to predict the number.
> One candidate is the sum of util_avg in this LLC domain. The benefit
> of choosing util_avg is that it is a metric of accumulated historic
> activity, which seems to be smoother than instantaneous metrics
> (such as rq->nr_running). Besides, choosing the sum of util_avg
> would help predict the load of the LLC domain more precisely, because
> SIS_PROP uses one CPU's idle time to estimate the total LLC domain idle
> time.
>
> In summary, the lower the util_avg is, the more select_idle_cpu()
> should scan for idle CPU, and vice versa. When the sum of util_avg
> in this LLC domain hits 85% or above, the scan stops. The reason to
> choose 85% as the threshold is that this is the imbalance_pct(117)
> when a LLC sched group is overloaded.
>
> Introduce the quadratic function:
>
> y = SCHED_CAPACITY_SCALE - p * x^2
> and y'= y / SCHED_CAPACITY_SCALE
>
> x is the ratio of sum_util compared to the CPU capacity:
> x = sum_util / (llc_weight * SCHED_CAPACITY_SCALE)
> y' is the ratio of CPUs to be scanned in the LLC domain,
> and the number of CPUs to scan is calculated by:
>
> nr_scan = llc_weight * y'
>
> Choosing quadratic function is because:
> [1] Compared to the linear function, it scans more aggressively when the
> sum_util is low.
> [2] Compared to the exponential function, it is easier to calculate.
> [3] It seems that there is no accurate mapping between the sum of util_avg
> and the number of CPUs to be scanned. Use heuristic scan for now.
>
> For a platform with 112 CPUs per LLC, the number of CPUs to scan is:
> sum_util% 0 5 15 25 35 45 55 65 75 85 86 ...
> scan_nr 112 111 108 102 93 81 65 47 25 1 0 ...
>
> For a platform with 16 CPUs per LLC, the number of CPUs to scan is:
> sum_util% 0 5 15 25 35 45 55 65 75 85 86 ...
> scan_nr 16 15 15 14 13 11 9 6 3 0 0 ...
>
> Furthermore, to minimize the overhead of calculating the metrics in
> select_idle_cpu(), borrow the statistics from periodic load balance.
> As mentioned by Abel, on a platform with 112 CPUs per LLC, the
> sum_util calculated by periodic load balance after 112 ms would
> decay to about 0.5 * 0.5 * 0.5 * 0.7 = 8.75%, thus bringing a delay
> in reflecting the latest utilization. But it is a trade-off.
> Checking the util_avg in newidle load balance would be more frequent,
> but it brings overhead - multiple CPUs write/read the per-LLC shared
> variable and introduces cache contention. Tim also mentioned that,
> it is allowed to be non-optimal in terms of scheduling for the
> short-term variations, but if there is a long-term trend in the load
> behavior, the scheduler can adjust for that.
>
> When SIS_UTIL is enabled, the select_idle_cpu() uses the nr_scan
> calculated by SIS_UTIL instead of the one from SIS_PROP. As Peter and
> Mel suggested, SIS_UTIL should be enabled by default.
>
> This patch is based on the util_avg, which is very sensitive to the
> CPU frequency invariance. There is an issue that, when the max frequency
> has been clamp, the util_avg would decay insanely fast when
> the CPU is idle. Commit addca285120b ("cpufreq: intel_pstate: Handle no_turbo
> in frequency invariance") could be used to mitigate this symptom, by adjusting
> the arch_max_freq_ratio when turbo is disabled. But this issue is still
> not thoroughly fixed, because the current code is unaware of the user-specified
> max CPU frequency.
>
> [Test result]
>
> netperf and tbench were launched with 25% 50% 75% 100% 125% 150%
> 175% 200% of CPU number respectively. Hackbench and schbench were launched
> by 1, 2 ,4, 8 groups. Each test lasts for 100 seconds and repeats 3 times.
>
> The following is the benchmark result comparison between
> baseline:vanilla v5.19-rc1 and compare:patched kernel. Positive compare%
> indicates better performance.
>
> Each netperf test is a:
> netperf -4 -H 127.0.1 -t TCP/UDP_RR -c -C -l 100
> netperf.throughput
> =======
> case load baseline(std%) compare%( std%)
> TCP_RR 28 threads 1.00 ( 0.34) -0.16 ( 0.40)
> TCP_RR 56 threads 1.00 ( 0.19) -0.02 ( 0.20)
> TCP_RR 84 threads 1.00 ( 0.39) -0.47 ( 0.40)
> TCP_RR 112 threads 1.00 ( 0.21) -0.66 ( 0.22)
> TCP_RR 140 threads 1.00 ( 0.19) -0.69 ( 0.19)
> TCP_RR 168 threads 1.00 ( 0.18) -0.48 ( 0.18)
> TCP_RR 196 threads 1.00 ( 0.16) +194.70 ( 16.43)
> TCP_RR 224 threads 1.00 ( 0.16) +197.30 ( 7.85)
> UDP_RR 28 threads 1.00 ( 0.37) +0.35 ( 0.33)
> UDP_RR 56 threads 1.00 ( 11.18) -0.32 ( 0.21)
> UDP_RR 84 threads 1.00 ( 1.46) -0.98 ( 0.32)
> UDP_RR 112 threads 1.00 ( 28.85) -2.48 ( 19.61)
> UDP_RR 140 threads 1.00 ( 0.70) -0.71 ( 14.04)
> UDP_RR 168 threads 1.00 ( 14.33) -0.26 ( 11.16)
> UDP_RR 196 threads 1.00 ( 12.92) +186.92 ( 20.93)
> UDP_RR 224 threads 1.00 ( 11.74) +196.79 ( 18.62)
>
> Take the 224 threads as an example, the SIS search metrics changes are
> illustrated below:
>
> vanilla patched
> 4544492 +237.5% 15338634 sched_debug.cpu.sis_domain_search.avg
> 38539 +39686.8% 15333634 sched_debug.cpu.sis_failed.avg
> 128300000 -87.9% 15551326 sched_debug.cpu.sis_scanned.avg
> 5842896 +162.7% 15347978 sched_debug.cpu.sis_search.avg
>
> There is -87.9% less CPU scans after patched, which indicates lower overhead.
> Besides, with this patch applied, there is -13% less rq lock contention
> in perf-profile.calltrace.cycles-pp._raw_spin_lock.raw_spin_rq_lock_nested
> .try_to_wake_up.default_wake_function.woken_wake_function.
> This might help explain the performance improvement - Because this patch allows
> the waking task to remain on the previous CPU, rather than grabbing other CPUs'
> lock.
>
> Each hackbench test is a:
> hackbench -g $job --process/threads --pipe/sockets -l 1000000 -s 100
> hackbench.throughput
> =========
> case load baseline(std%) compare%( std%)
> process-pipe 1 group 1.00 ( 1.29) +0.57 ( 0.47)
> process-pipe 2 groups 1.00 ( 0.27) +0.77 ( 0.81)
> process-pipe 4 groups 1.00 ( 0.26) +1.17 ( 0.02)
> process-pipe 8 groups 1.00 ( 0.15) -4.79 ( 0.02)
> process-sockets 1 group 1.00 ( 0.63) -0.92 ( 0.13)
> process-sockets 2 groups 1.00 ( 0.03) -0.83 ( 0.14)
> process-sockets 4 groups 1.00 ( 0.40) +5.20 ( 0.26)
> process-sockets 8 groups 1.00 ( 0.04) +3.52 ( 0.03)
> threads-pipe 1 group 1.00 ( 1.28) +0.07 ( 0.14)
> threads-pipe 2 groups 1.00 ( 0.22) -0.49 ( 0.74)
> threads-pipe 4 groups 1.00 ( 0.05) +1.88 ( 0.13)
> threads-pipe 8 groups 1.00 ( 0.09) -4.90 ( 0.06)
> threads-sockets 1 group 1.00 ( 0.25) -0.70 ( 0.53)
> threads-sockets 2 groups 1.00 ( 0.10) -0.63 ( 0.26)
> threads-sockets 4 groups 1.00 ( 0.19) +11.92 ( 0.24)
> threads-sockets 8 groups 1.00 ( 0.08) +4.31 ( 0.11)
>
> Each tbench test is a:
> tbench -t 100 $job 127.0.0.1
> tbench.throughput
> ======
> case load baseline(std%) compare%( std%)
> loopback 28 threads 1.00 ( 0.06) -0.14 ( 0.09)
> loopback 56 threads 1.00 ( 0.03) -0.04 ( 0.17)
> loopback 84 threads 1.00 ( 0.05) +0.36 ( 0.13)
> loopback 112 threads 1.00 ( 0.03) +0.51 ( 0.03)
> loopback 140 threads 1.00 ( 0.02) -1.67 ( 0.19)
> loopback 168 threads 1.00 ( 0.38) +1.27 ( 0.27)
> loopback 196 threads 1.00 ( 0.11) +1.34 ( 0.17)
> loopback 224 threads 1.00 ( 0.11) +1.67 ( 0.22)
>
> Each schbench test is a:
> schbench -m $job -t 28 -r 100 -s 30000 -c 30000
> schbench.latency_90%_us
> ========
> case load baseline(std%) compare%( std%)
> normal 1 mthread 1.00 ( 31.22) -7.36 ( 20.25)*
> normal 2 mthreads 1.00 ( 2.45) -0.48 ( 1.79)
> normal 4 mthreads 1.00 ( 1.69) +0.45 ( 0.64)
> normal 8 mthreads 1.00 ( 5.47) +9.81 ( 14.28)
>
> *Consider the Standard Deviation, this -7.36% regression might not be valid.
>
> Also, a OLTP workload with a commercial RDBMS has been tested, and there
> is no significant change.
>
> There were concerns that unbalanced tasks among CPUs would cause problems.
> For example, suppose the LLC domain is composed of 8 CPUs, and 7 tasks are
> bound to CPU0~CPU6, while CPU7 is idle:
>
> CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
> util_avg 1024 1024 1024 1024 1024 1024 1024 0
>
> Since the util_avg ratio is 87.5%( = 7/8 ), which is higher than 85%,
> select_idle_cpu() will not scan, thus CPU7 is undetected during scan.
> But according to Mel, it is unlikely the CPU7 will be idle all the time
> because CPU7 could pull some tasks via CPU_NEWLY_IDLE.
>
> lkp(kernel test robot) has reported a regression on stress-ng.sock on a
> very busy system. According to the sched_debug statistics, it might be caused
> by SIS_UTIL terminates the scan and chooses a previous CPU earlier, and this
> might introduce more context switch, especially involuntary preemption, which
> impacts a busy stress-ng. This regression has shown that, not all benchmarks
> in every scenario benefit from idle CPU scan limit, and it needs further
> investigation.
>
> Besides, there is slight regression in hackbench's 16 groups case when the
> LLC domain has 16 CPUs. Prateek mentioned that we should scan aggressively
> in an LLC domain with 16 CPUs. Because the cost to search for an idle one
> among 16 CPUs is negligible. The current patch aims to propose a generic
> solution and only considers the util_avg. Something like the below could
> be applied on top of the current patch to fulfill the requirement:
>
> if (llc_weight <= 16)
> nr_scan = nr_scan * 32 / llc_weight;
>
> For LLC domain with 16 CPUs, the nr_scan will be expanded to 2 times large.
> The smaller the CPU number this LLC domain has, the larger nr_scan will be
> expanded. This needs further investigation.
>
> There is also ongoing work[2] from Abel to filter out the busy CPUs during
> wakeup, to further speed up the idle CPU scan. And it could be a following-up
> optimization on top of this change.
>
> v3->v4:
> No fundamental change since v3.
> - Enable SIS_UTIL by default. (Peter Zijlstra, Mel Gorman)
> - Replace percentage with SCHED_CAPACITY_SCALE based calculation.
> (Peter Zijlstra)
> - Use imbalance_pct for threshold rather than hardcoded of 85%.
> (Mel Gorman)
> - Add description of the patch frequency invariance dependence in
> changelog.(Mel Gorman)
> - Remove the inline of update_idle_cpu_scan(). (Mel Gorman)
> - Move the check of CPU_NEWLY_IDLE earlier, to avoid unnecessary
> percpu cache contention. (Mel Gorman)
> - Add comments on why CPU_NEWLY_IDLE is ignored in update_idle_cpu_scan(),
> because updating sd_shared which is a shared cache line write and
> CPU_NEWLY_IDLE can fire way more frequently than periodic load
> balancing. (Mel Gorman)
> - Rename nr_llc to llc_weight to avoid confusion. (Mel Gorman)
> - Avoid writing the same value to sd_share->nr_idle_scan to reduce
> cache line bounces. (Mel Gorman)
>
> v2->v3:
> - Use 85% as the threshold again, because the CPU frequency invariance issue
> has been fixed and the patch is queued for 5.19.
>
> - Stop the scan if 85% is reached, rather than scanning for at least 4 CPUs.
> (Yicong Yang)
>
> - Replace linear scan with quadratic function scan, to let the SIS scan
> aggressively when the LLC is not busy. Prateek mentioned there was slight
> regression from ycsb-mongodb in v2, which might be due to fewer CPUs
> scanned when the utilization is around 20%. (K Prateek Nayak)
>
> - Add back the logic to stop the CPU scan even if has_idle_core is true.
> It might be a waste of time to search for an idle Core if the LLC is
> overloaded. Besides, according to the tbench result from Prateek, stop idle
> Core scan would bring extra performance improvement. (K Prateek Nayak)
>
> - Provide the SIS search statistics in the commit log, based on Mel Gorman's
> patch. (Abel Wu)
>
> - Introduce SIS_UTIL sched feature rather than changing the logic of SIS_PROP
> directly, which can be reviewed easier.
>
> v2->v1:
> - As suggested by Peter, introduce an idle CPU scan strategy that is based on
> the util_avg metric. When util_avg is very low it scans more, while when
> util_avg hits the threshold we naturally stop scanning entirely. The threshold
> has been decreased from 85% to 50%, because this is the threshold when the
> CPU is nearly 100% but with turbo disabled. At least scan for 4 CPUs even
> when the LLC is overloaded, to keep it consistent with the current logic of
> select_idle_cpu().
>
> v1:
> - Stop scanning the idle CPU in select_idle_cpu(), if the sum of util_avg in
> the LLC domain has reached 85%.
>
> Link: https://lore.kernel.org/lkml/20210726102247.21437-2-mgorman@techsingularity.net #1
> Link: https://lore.kernel.org/lkml/20220505122331.42696-1-wuyun.abel@bytedance.com #2
> Suggested-by: Tim Chen <tim.c.chen@intel.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Tested-by: Yicong Yang <yangyicong@hisilicon.com>
> Tested-by: Mohini Narkhede <mohini.narkhede@intel.com>
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> ---
> include/linux/sched/topology.h | 1 +
> kernel/sched/fair.c | 87 ++++++++++++++++++++++++++++++++++
> kernel/sched/features.h | 1 +
> 3 files changed, 89 insertions(+)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 56cffe42abbc..816df6cc444e 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -81,6 +81,7 @@ struct sched_domain_shared {
> atomic_t ref;
> atomic_t nr_busy_cpus;
> int has_idle_cores;
> + int nr_idle_scan;
> };
>
> struct sched_domain {
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 77b2048a9326..3fb857a35b16 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6336,6 +6336,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> {
> struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> int i, cpu, idle_cpu = -1, nr = INT_MAX;
> + struct sched_domain_shared *sd_share;
> struct rq *this_rq = this_rq();
> int this = smp_processor_id();
> struct sched_domain *this_sd;
> @@ -6375,6 +6376,17 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> time = cpu_clock(this);
> }
>
> + if (sched_feat(SIS_UTIL)) {
> + sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
> + if (sd_share) {
> + /* because !--nr is the condition to stop scan */
> + nr = READ_ONCE(sd_share->nr_idle_scan) + 1;
> + /* overloaded LLC is unlikely to have idle cpu/core */
> + if (nr == 1)
> + return -1;
> + }
> + }
> +
> for_each_cpu_wrap(cpu, cpus, target + 1) {
> if (has_idle_core) {
> i = select_idle_core(p, cpu, cpus, &idle_cpu);
> @@ -9222,6 +9234,77 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> return idlest;
> }
>
> +static void update_idle_cpu_scan(struct lb_env *env,
> + unsigned long sum_util)
> +{
> + struct sched_domain_shared *sd_share;
> + int llc_weight, pct;
> + u64 x, y, tmp;
> + /*
> + * Update the number of CPUs to scan in LLC domain, which could
> + * be used as a hint in select_idle_cpu(). The update of sd_share
> + * could be expensive because it is within a shared cache line.
> + * So the write of this hint only occurs during periodic load
> + * balancing, rather than CPU_NEWLY_IDLE, because the latter
> + * can fire way more frequently than the former.
> + */
> + if (!sched_feat(SIS_UTIL) || env->idle == CPU_NEWLY_IDLE)
> + return;
> +
> + llc_weight = per_cpu(sd_llc_size, env->dst_cpu);
> + if (env->sd->span_weight != llc_weight)
> + return;
> +
> + sd_share = rcu_dereference(per_cpu(sd_llc_shared, env->dst_cpu));
> + if (!sd_share)
> + return;
> +
> + /*
> + * The number of CPUs to search drops as sum_util increases, when
> + * sum_util hits 85% or above, the scan stops.
> + * The reason to choose 85% as the threshold is because this is the
> + * imbalance_pct(117) when a LLC sched group is overloaded.
> + *
> + * let y = SCHED_CAPACITY_SCALE - p * x^2 [1]
> + * and y'= y / SCHED_CAPACITY_SCALE
> + *
> + * x is the ratio of sum_util compared to the CPU capacity:
> + * x = sum_util / (llc_weight * SCHED_CAPACITY_SCALE)
> + * y' is the ratio of CPUs to be scanned in the LLC domain,
> + * and the number of CPUs to scan is calculated by:
> + *
> + * nr_scan = llc_weight * y' [2]
> + *
> + * When x hits the threshold of overloaded, AKA, when
> + * x = 100 / pct, y drops to 0. According to [1],
> + * p should be SCHED_CAPACITY_SCALE * pct^2 / 10000
> + *
> + * Scale x by SCHED_CAPACITY_SCALE:
> + * x' = sum_util / llc_weight; [3]
> + *
> + * and finally [1] becomes:
> + * y = SCHED_CAPACITY_SCALE -
> + * x'^2 * pct^2 / (10000 * SCHED_CAPACITY_SCALE) [4]
> + *
> + */
> + /* equation [3] */
> + x = sum_util;
> + do_div(x, llc_weight);
> +
> + /* equation [4] */
> + pct = env->sd->imbalance_pct;
> + tmp = x * x * pct * pct;
> + do_div(tmp, 10000 * SCHED_CAPACITY_SCALE);
> + tmp = min_t(long, tmp, SCHED_CAPACITY_SCALE);
> + y = SCHED_CAPACITY_SCALE - tmp;
> +
> + /* equation [2] */
> + y *= llc_weight;
> + do_div(y, SCHED_CAPACITY_SCALE);
> + if ((int)y != sd_share->nr_idle_scan)
> + WRITE_ONCE(sd_share->nr_idle_scan, (int)y);
> +}
> +
> /**
> * update_sd_lb_stats - Update sched_domain's statistics for load balancing.
> * @env: The load balancing environment.
> @@ -9234,6 +9317,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> struct sched_group *sg = env->sd->groups;
> struct sg_lb_stats *local = &sds->local_stat;
> struct sg_lb_stats tmp_sgs;
> + unsigned long sum_util = 0;
> int sg_status = 0;
>
> do {
> @@ -9266,6 +9350,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> sds->total_load += sgs->group_load;
> sds->total_capacity += sgs->group_capacity;
>
> + sum_util += sgs->group_util;
> sg = sg->next;
> } while (sg != env->sd->groups);
>
> @@ -9291,6 +9376,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> WRITE_ONCE(rd->overutilized, SG_OVERUTILIZED);
> trace_sched_overutilized_tp(rd, SG_OVERUTILIZED);
> }
> +
> + update_idle_cpu_scan(env, sum_util);
> }
>
> #define NUMA_IMBALANCE_MIN 2
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 1cf435bbcd9c..3334a1b93fc6 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -61,6 +61,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
> * When doing wakeups, attempt to limit superfluous scans of the LLC domain.
> */
> SCHED_FEAT(SIS_PROP, true)
> +SCHED_FEAT(SIS_UTIL, true)
>

confused here that shouldn't we have SCHED_FEAT(SIS_PROP, false)? With SIS_UTIL enabled, SIS_PROP will have no
effect since nr is overridden by SIS_UTIL.

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