Messages in this thread | | | From | Vitalii Bursov <> | Subject | [PATCH 0/1] sched/fair: allow disabling newidle_balance with sched_relax_domain_level | Date | Thu, 28 Mar 2024 02:30:17 +0200 |
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Hi,
During the upgrade from Linux 5.4 we found a small (around 3%) performance regression which was tracked to commit c5b0a7eefc70150caf23e37bc9d639c68c87a097
sched/fair: Remove sysctl_sched_migration_cost condition
With a default value of 500us, sysctl_sched_migration_cost is significanlty higher than the cost of load_balance. Remove the condition and rely on the sd->max_newidle_lb_cost to abort newidle_balance.
https://lore.kernel.org/lkml/20211019123537.17146-6-vincent.guittot@linaro.org/T/#m248877858373980d8fffa7156931211a1f0d7c4c
Looks like "newidle" balancing is beneficial for a lot of workloads, just not for this specific one. The workload is video encoding, there are 100s-1000s of threads, some are synchonized with mutexes and conditional variables. The process aims to have a portion of CPU idle, so no CPU cores are 100% busy. Perhaps, the performance impact we see comes from additional processing in the scheduler and additional cost like more cache misses, and not from an incorrect balancing. See perf output below.
My understanding is that "sched_relax_domain_level" cgroup parameter should control if newidle_balance() is called and what's the scope of the balancing is, but it doesn't fully work for this case.
cpusets.rst documentation: > The 'cpuset.sched_relax_domain_level' file allows you to request changing > this searching range as you like. This file takes int value which > indicates size of searching range in levels ideally as follows, > otherwise initial value -1 that indicates the cpuset has no request. > > ====== =========================================================== > -1 no request. use system default or follow request of others. > 0 no search. > 1 search siblings (hyperthreads in a core). > 2 search cores in a package. > 3 search cpus in a node [= system wide on non-NUMA system] > 4 search nodes in a chunk of node [on NUMA system] > 5 search system wide [on NUMA system] > ====== ===========================================================
Setting cpuset.sched_relax_domain_level to 0 works as 1.
On a dual-CPU server, domains and levels are as follows: domain 0: level 0, SMT domain 1: level 2, MC domain 2: level 5, NUMA
So, to support "0 no search", the value in cpuset.sched_relax_domain_level should disable SD_BALANCE_NEWIDLE for a specified level and keep it enabled for prior levels. For example, SMT level is 0, so sched_relax_domain_level=0 should exclude levels >=0.
Instead, cpuset.sched_relax_domain_level enables the specified level, which effectively removes "no search" option. See below for domain flags for all cpuset.sched_relax_domain_level values.
Proposed patch allows clearing SD_BALANCE_NEWIDLE flags when cpuset.sched_relax_domain_level is set to 0 and extends max value validation range beyond sched_domain_level_max. This allows setting SD_BALANCE_NEWIDLE on all levels and override platform default if it does not include all levels.
Thanks
========================= Perf output for a simimar workload/test case shows that newidle_balance is called when handling futex and nanosleep syscalls: 8.74% 0.40% a.out [kernel.vmlinux] [k] entry_SYSCALL_64 8.34% entry_SYSCALL_64 - do_syscall_64 - 5.50% __x64_sys_futex - 5.42% do_futex - 3.79% futex_wait - 3.74% __futex_wait - 3.53% futex_wait_queue - 3.45% schedule - 3.43% __schedule - 2.06% pick_next_task - 1.93% pick_next_task_fair - 1.87% newidle_balance - 1.52% load_balance - 1.16% find_busiest_group - 1.13% update_sd_lb_stats.constprop.0 1.01% update_sg_lb_stats - 0.83% dequeue_task_fair 0.66% dequeue_entity - 1.57% futex_wake - 1.22% wake_up_q - 1.20% try_to_wake_up 0.58% select_task_rq_fair - 2.44% __x64_sys_nanosleep - 2.36% hrtimer_nanosleep - 2.33% do_nanosleep - 2.05% schedule - 2.03% __schedule - 1.23% pick_next_task - 1.15% pick_next_task_fair - 1.12% newidle_balance - 0.90% load_balance - 0.68% find_busiest_group - 0.66% update_sd_lb_stats.constprop.0 0.59% update_sg_lb_stats 0.52% dequeue_task_fair
When newidle_balance is disabled (or when using older kernels), perf output is: 6.37% 0.41% a.out [kernel.vmlinux] [k] entry_SYSCALL_64 5.96% entry_SYSCALL_64 - do_syscall_64 - 3.97% __x64_sys_futex - 3.89% do_futex - 2.32% futex_wait - 2.27% __futex_wait - 2.05% futex_wait_queue - 1.98% schedule - 1.96% __schedule - 0.81% dequeue_task_fair 0.66% dequeue_entity - 0.64% pick_next_task 0.51% pick_next_task_fair - 1.52% futex_wake - 1.15% wake_up_q - try_to_wake_up 0.59% select_task_rq_fair - 1.58% __x64_sys_nanosleep - 1.52% hrtimer_nanosleep - 1.48% do_nanosleep - 1.20% schedule - 1.19% __schedule 0.53% dequeue_task_fair
Without a patch: ========================= CPUs: 2 Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
# uname -r 6.8.1
# numactl -H available: 2 nodes (0-1) node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 24 25 26 27 28 29 30 31 32 33 34 35 node 0 size: 63962 MB node 0 free: 59961 MB node 1 cpus: 12 13 14 15 16 17 18 19 20 21 22 23 36 37 38 39 40 41 42 43 44 45 46 47 node 1 size: 64446 MB node 1 free: 63338 MB node distances: node 0 1 0: 10 21 1: 21 10
# head /proc/schedstat version 15 timestamp 4295347219 cpu0 0 0 0 0 0 0 3035466036 858375615 67578 domain0 0000,01000001 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0... domain1 000f,ff000fff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0... domain2 ffff,ffffffff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0...
# cd /sys/kernel/debug/sched/domains # echo -1 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{name,flags,groups_flags,max_newidle_lb_cost} cpu0/domain0/name:SMT cpu0/domain1/name:MC cpu0/domain2/name:NUMA
cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain0/max_newidle_lb_cost:2236 cpu0/domain1/max_newidle_lb_cost:3444 cpu0/domain2/max_newidle_lb_cost:4590
# echo 0 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags,max_newidle_lb_cost} cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain1/flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/groups_flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain0/max_newidle_lb_cost:0 cpu0/domain1/max_newidle_lb_cost:0 cpu0/domain2/max_newidle_lb_cost:0
# echo 1 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags,max_newidle_lb_cost}
cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain1/flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/groups_flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain0/max_newidle_lb_cost:309 cpu0/domain1/max_newidle_lb_cost:0 cpu0/domain2/max_newidle_lb_cost:0
# echo 2 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags,max_newidle_lb_cost}
cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain0/max_newidle_lb_cost:276 cpu0/domain1/max_newidle_lb_cost:2776 cpu0/domain2/max_newidle_lb_cost:0
# echo 3 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags,max_newidle_lb_cost} cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain0/max_newidle_lb_cost:289 cpu0/domain1/max_newidle_lb_cost:3192 cpu0/domain2/max_newidle_lb_cost:0
# echo 4 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags,max_newidle_lb_cost} cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/flags:SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_CPUCAPACITY SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain2/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING cpu0/domain0/max_newidle_lb_cost:1306 cpu0/domain1/max_newidle_lb_cost:1999 cpu0/domain2/max_newidle_lb_cost:0
# echo 5 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level bash: echo: write error: Invalid argument =========================
The same system with the patch applied: ========================= # cd /sys/kernel/debug/sched/domains # echo -1 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{name,level,flags,groups_flags} cpu0/domain0/name:SMT cpu0/domain1/name:MC cpu0/domain2/name:NUMA cpu0/domain0/level:0 cpu0/domain1/level:2 cpu0/domain2/level:5 cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain2/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ...
# echo 0 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags} cpu0/domain0/flags:SD_BALANCE_EXEC ... cpu0/domain1/flags:SD_BALANCE_EXEC ... cpu0/domain2/flags:SD_BALANCE_EXEC ... cpu0/domain1/groups_flags:SD_BALANCE_EXEC ... cpu0/domain2/groups_flags:SD_BALANCE_EXEC ...
# echo 1 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags} cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain1/flags:SD_BALANCE_EXEC ... cpu0/domain2/flags:SD_BALANCE_EXEC ... cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain2/groups_flags:SD_BALANCE_EXEC ...
[skip 2, same as 1]
# echo 3 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags} cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain2/flags:SD_BALANCE_EXEC ... cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain2/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ...
[skip 4 and 5, same as 3]
# echo 6 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level # grep . cpu0/*/{flags,groups_flags} cpu0/domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain1/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ... cpu0/domain2/groups_flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC ...
# echo 7 > /sys/fs/cgroup/cpuset/cpuset.sched_relax_domain_level bash: echo: write error: Invalid argument =========================
Vitalii Bursov (1): sched/fair: allow disabling newidle_balance with sched_relax_domain_level
kernel/cgroup/cpuset.c | 2 +- kernel/sched/debug.c | 1 + kernel/sched/topology.c | 2 +- 3 files changed, 3 insertions(+), 2 deletions(-)
-- 2.20.1
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