From 22d5607400c62c72da9b60e3324744be83e147a4 Mon Sep 17 00:00:00 2001 From: Qais Yousef Date: Sun, 24 Mar 2024 00:45:50 +0000 Subject: sched/fair: Check if a task has a fitting CPU when updating misfit If a misfit task is affined to a subset of the possible CPUs, we need to verify that one of these CPUs can fit it. Otherwise the load balancer code will continuously trigger needlessly leading the balance_interval to increase in return and eventually end up with a situation where real imbalances take a long time to address because of this impossible imbalance situation. This can happen in Android world where it's common for background tasks to be restricted to little cores. Similarly if we can't fit the biggest core, triggering misfit is pointless as it is the best we can ever get on this system. To be able to detect that; we use asym_cap_list to iterate through capacities in the system to see if the task is able to run at a higher capacity level based on its p->cpus_ptr. We do that when the affinity change, a fair task is forked, or when a task switched to fair policy. We store the max_allowed_capacity in task_struct to allow for cheap comparison in the fast path. Improve check_misfit_status() function by removing redundant checks. misfit_task_load will be 0 if the task can't move to a bigger CPU. And nohz_balancer_kick() already checks for cpu_check_capacity() before calling check_misfit_status(). Test: ===== Add trace_printk("balance_interval = %lu\n", interval) in get_sd_balance_interval(). run if [ "$MASK" != "0" ]; then adb shell "taskset -a $MASK cat /dev/zero > /dev/null" fi sleep 10 // parse ftrace buffer counting the occurrence of each valaue Where MASK is either: * 0: no busy task running * 1: busy task is pinned to 1 cpu; handled today to not cause misfit * f: busy task pinned to little cores, simulates busy background task, demonstrates the problem to be fixed Results: ======== Note how occurrence of balance_interval = 128 overshoots for MASK = f. BEFORE ------ MASK=0 1 balance_interval = 175 120 balance_interval = 128 846 balance_interval = 64 55 balance_interval = 63 215 balance_interval = 32 2 balance_interval = 31 2 balance_interval = 16 4 balance_interval = 8 1870 balance_interval = 4 65 balance_interval = 2 MASK=1 27 balance_interval = 175 37 balance_interval = 127 840 balance_interval = 64 167 balance_interval = 63 449 balance_interval = 32 84 balance_interval = 31 304 balance_interval = 16 1156 balance_interval = 8 2781 balance_interval = 4 428 balance_interval = 2 MASK=f 1 balance_interval = 175 1328 balance_interval = 128 44 balance_interval = 64 101 balance_interval = 63 25 balance_interval = 32 5 balance_interval = 31 23 balance_interval = 16 23 balance_interval = 8 4306 balance_interval = 4 177 balance_interval = 2 AFTER ----- Note how the high values almost disappear for all MASK values. The system has background tasks that could trigger the problem without simulate it even with MASK=0. MASK=0 103 balance_interval = 63 19 balance_interval = 31 194 balance_interval = 8 4827 balance_interval = 4 179 balance_interval = 2 MASK=1 131 balance_interval = 63 1 balance_interval = 31 87 balance_interval = 8 3600 balance_interval = 4 7 balance_interval = 2 MASK=f 8 balance_interval = 127 182 balance_interval = 63 3 balance_interval = 31 9 balance_interval = 16 415 balance_interval = 8 3415 balance_interval = 4 21 balance_interval = 2 Signed-off-by: Qais Yousef Signed-off-by: Ingo Molnar Reviewed-by: Vincent Guittot Link: https://lore.kernel.org/r/20240324004552.999936-3-qyousef@layalina.io --- init/init_task.c | 1 + 1 file changed, 1 insertion(+) (limited to 'init') diff --git a/init/init_task.c b/init/init_task.c index 4daee6d761c8..2558b719e053 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -77,6 +77,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = { .cpus_ptr = &init_task.cpus_mask, .user_cpus_ptr = NULL, .cpus_mask = CPU_MASK_ALL, + .max_allowed_capacity = SCHED_CAPACITY_SCALE, .nr_cpus_allowed= NR_CPUS, .mm = NULL, .active_mm = &init_mm, -- cgit v1.2.3