From nobody Sun Feb 8 18:27:42 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 65A66C77B73 for ; Mon, 5 Jun 2023 19:04:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234726AbjFETEL (ORCPT ); Mon, 5 Jun 2023 15:04:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59354 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233162AbjFETDm (ORCPT ); Mon, 5 Jun 2023 15:03:42 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C4717ED for ; Mon, 5 Jun 2023 12:02:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1685991774; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: references:references; bh=IAf4R5o0gQZjozbQ8tE/X+XgLQMuANKlfa1dzEwKui8=; b=Tt5gns3cK2TH3PU7w2gLIfATM2RrpHK/d1Sfdf0X8Vc6Kmc9YUIidfKVCeJiQ8lVW3Z7Cs 5J/sVeEZRNZktqlFg6QoBrQ0YKs+HZRtxtZesF/Nh3t+RtZd6DCbNMYpu2qJEX/1Ln6lSw aYX2tV6UMJNMrxFXI4MnUCCXbkfF8AM= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-517-z2HfVbExNb-4TD6HCY8g_Q-1; Mon, 05 Jun 2023 15:02:51 -0400 X-MC-Unique: z2HfVbExNb-4TD6HCY8g_Q-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 69CB38039B1; Mon, 5 Jun 2023 19:02:50 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-3.gru2.redhat.com [10.97.112.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3196240D1B61; Mon, 5 Jun 2023 19:02:50 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 39AF6400F7B4D; Mon, 5 Jun 2023 16:02:31 -0300 (-03) Message-ID: <20230605190132.059270652@redhat.com> User-Agent: quilt/0.67 Date: Mon, 05 Jun 2023 15:56:29 -0300 From: Marcelo Tosatti To: Christoph Lameter Cc: Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Vlastimil Babka , Michal Hocko , Marcelo Tosatti Subject: [PATCH v3 2/3] vmstat: skip periodic vmstat update for isolated CPUs References: <20230605185627.923698377@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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=20 (use case: soft PLC). oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000) oslat 1094.456971: workqueue_queue_work: ... function=3Dvmstat_update ... oslat 1094.456974: sched_switch: prev_comm=3Doslat ... =3D=3D> next_comm= =3Dkworker/5:1 ... kworker 1094.456978: sched_switch: prev_comm=3Dkworker/5:1 =3D=3D> next_com= m=3Doslat ... The example above shows an additional 7us for the = = = = =20 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: =3D> 0xffffffffc042b083 =3D> mod_zone_page_state =3D> __folio_end_writeback =3D> folio_end_writeback =3D> iomap_finish_ioend =3D> blk_mq_end_request_batch =3D> nvme_irq =3D> __handle_irq_event_percpu =3D> handle_irq_event =3D> handle_edge_irq =3D> __common_interrupt =3D> common_interrupt =3D> asm_common_interrupt =3D> vmx_do_interrupt_nmi_irqoff =3D> vmx_handle_exit_irqoff =3D> vcpu_enter_guest =3D> vcpu_run =3D> kvm_arch_vcpu_ioctl_run =3D> kvm_vcpu_ioctl =3D> __x64_sys_ioctl =3D> do_syscall_64 =3D> 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. Acked-by: Michal Hocko Signed-off-by: Marcelo Tosatti --- v3: improve changelog (Michal Hocko) v2: use cpu_is_isolated (Michal Hocko) Index: linux-vmstat-remote/mm/vmstat.c =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D --- linux-vmstat-remote.orig/mm/vmstat.c +++ linux-vmstat-remote/mm/vmstat.c @@ -28,6 +28,7 @@ #include #include #include +#include =20 #include "internal.h" =20 @@ -2022,6 +2023,20 @@ static void vmstat_shepherd(struct work_ for_each_online_cpu(cpu) { struct delayed_work *dw =3D &per_cpu(vmstat_work, cpu); =20 + /* + * In kernel users of vmstat counters either require the precise value a= nd + * they are using zone_page_state_snapshot interface or they can live wi= th + * an imprecision as the regular flushing can happen at arbitrary time a= nd + * 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_sheph= erd + * for all isolated CPUs to avoid interference with the isolated workloa= d. + */ + if (cpu_is_isolated(cpu)) + continue; + if (!delayed_work_pending(dw) && need_update(cpu)) queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);