From be5e015d107d5336f298b74ea5a4f0b1773bc6f9 Mon Sep 17 00:00:00 2001 From: Marcelo Tosatti Date: Wed, 7 Jun 2023 17:28:07 -0300 Subject: [PATCH] vmstat: skip periodic vmstat update for isolated CPUs Problem: The interruption caused by vmstat_update is undesirable for certain applications. With workloads that are running on isolated cpus with nohz full mode to shield off any kernel interruption. For example, a VM running a time sensitive application with a 50us maximum acceptable interruption (use case: soft PLC). oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000) oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ... oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ... kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ... The example above shows an additional 7us for the oslat -> kworker -> oslat switches. In the case of a virtualized CPU, and the vmstat_update interruption in the host (of a qemu-kvm vcpu), the latency penalty observed in the guest is higher than 50us, violating the acceptable latency threshold. The isolated vCPU can perform operations that modify per-CPU page counters, for example to complete I/O operations: CPU 11/KVM-9540 [001] dNh1. 2314.248584: mod_zone_page_state <-__folio_end_writeback CPU 11/KVM-9540 [001] dNh1. 2314.248585: => 0xffffffffc042b083 => mod_zone_page_state => __folio_end_writeback => folio_end_writeback => iomap_finish_ioend => blk_mq_end_request_batch => nvme_irq => __handle_irq_event_percpu => handle_irq_event => handle_edge_irq => __common_interrupt => common_interrupt => asm_common_interrupt => vmx_do_interrupt_nmi_irqoff => vmx_handle_exit_irqoff => vcpu_enter_guest => vcpu_run => kvm_arch_vcpu_ioctl_run => kvm_vcpu_ioctl => __x64_sys_ioctl => do_syscall_64 => entry_SYSCALL_64_after_hwframe In kernel users of vmstat counters either require the precise value and they are using zone_page_state_snapshot interface or they can live with an imprecision as the regular flushing can happen at arbitrary time and cumulative error can grow (see calculate_normal_threshold). From that POV the regular flushing can be postponed for CPUs that have been isolated from the kernel interference without critical infrastructure ever noticing. Skip regular flushing from vmstat_shepherd for all isolated CPUs to avoid interference with the isolated workload. Suggested by Michal Hocko. Link: https://lkml.kernel.org/r/ZIDoV/zxFKVmQl7W@tpad Signed-off-by: Marcelo Tosatti Acked-by: Michal Hocko Cc: Frederic Weisbecker Cc: Vlastimil Babka Signed-off-by: Andrew Morton --- mm/vmstat.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/mm/vmstat.c b/mm/vmstat.c index c28046371b45..c7499e3ee9d5 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -28,6 +28,7 @@ #include #include #include +#include #include "internal.h" @@ -2022,6 +2023,20 @@ static void vmstat_shepherd(struct work_struct *w) for_each_online_cpu(cpu) { struct delayed_work *dw = &per_cpu(vmstat_work, cpu); + /* + * In kernel users of vmstat counters either require the precise value and + * they are using zone_page_state_snapshot interface or they can live with + * an imprecision as the regular flushing can happen at arbitrary time and + * cumulative error can grow (see calculate_normal_threshold). + * + * From that POV the regular flushing can be postponed for CPUs that have + * been isolated from the kernel interference without critical + * infrastructure ever noticing. Skip regular flushing from vmstat_shepherd + * for all isolated CPUs to avoid interference with the isolated workload. + */ + if (cpu_is_isolated(cpu)) + continue; + if (!delayed_work_pending(dw) && need_update(cpu)) queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0); -- 2.11.0