From nobody Fri Dec 19 06:34:32 2025 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 76967C7EE25 for ; Thu, 8 Jun 2023 01:12:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232918AbjFHBMf (ORCPT ); Wed, 7 Jun 2023 21:12:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41560 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230418AbjFHBMd (ORCPT ); Wed, 7 Jun 2023 21:12:33 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 61E6E269F for ; Wed, 7 Jun 2023 18:11:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1686186705; 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; bh=mdQ0lF7C7zgaRMoTdeqNXNG4mPlYrnhL6rmrdXZfaoI=; b=XZmp0ACewuoRonIncg5Uzv8nIIX0l73ii6rj0oYTpleAgsyAl+F0lpq8hZme1AYS9MpXo7 rOfjs8R4t8IUMKCVdDjKa2SAeqDswcPMTrL/B7EFLDqwLR4pAooyzuLqbSIptLGpkym0Xh YVkuPuGdryIm35U3OPg4sG8ehmiK2Tc= 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-124-y1ZFZL7DPKeBSvgVD9UArA-1; Wed, 07 Jun 2023 21:11:42 -0400 X-MC-Unique: y1ZFZL7DPKeBSvgVD9UArA-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id D75D5185A78E; Thu, 8 Jun 2023 01:11:41 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A20F6403367; Thu, 8 Jun 2023 01:11:41 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id C29CC40E16DC2; Wed, 7 Jun 2023 17:28:07 -0300 (-03) Date: Wed, 7 Jun 2023 17:28:07 -0300 From: Marcelo Tosatti To: Andrew Morton Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Vlastimil Babka , Michal Hocko Subject: [PATCH] vmstat: skip periodic vmstat update for isolated CPUs Message-ID: MIME-Version: 1.0 Content-Disposition: inline X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 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 (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 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 = = = =20 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);