From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joe Buehler <aspam@cox.net>
Subject: probable vm exit latency issue?
Date: Fri, 15 Dec 2017 14:36:56 -0500
Message-ID: <5A342458.9010403@cox.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
To: kvm@vger.kernel.org
Return-path: <kvm-owner@vger.kernel.org>
Received: from fed1rmwml113.cox.net ([68.230.241.20]:49344 "EHLO
        fed1rmwml113.cox.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1755419AbdLOTmg (ORCPT <rfc822;kvm@vger.kernel.org>);
        Fri, 15 Dec 2017 14:42:36 -0500
Received: from eastrmimpo209.cox.net ([68.230.241.224])
          by eastrmfepo203.cox.net
          (InterMail vM.8.01.05.28 201-2260-151-171-20160122) with ESMTP
          id <20171215193700.NERZ4081.eastrmfepo203.cox.net@eastrmimpo209.cox.net>
          for <kvm@vger.kernel.org>; Fri, 15 Dec 2017 14:37:00 -0500
Received: from [10.10.10.15] (thunder.sweets [10.10.10.15])
        (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits))
        (No client certificate requested)
        by thunder.sweets (Postfix) with ESMTP id 997B7111B4
        for <kvm@vger.kernel.org>; Fri, 15 Dec 2017 14:36:59 -0500 (EST)
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

I am debugging large latency issues in a realtime KVM instance, trying to make an existing VM (debugged on another type of box) run on an HP proliant now.

cyclictest came up with the following snippet:

kworker/-71      0.....11 13483223us : ata_sff_exec_command <-ata_bmdma_setup
kworker/-71      0.....11 13483228us : ata_sff_sync <-ata_sff_exec_command
kworker/-71      0.....11 13483231us : delay_tsc <-__const_udelay
kworker/-71      0.....11 13483231us*: ata_bmdma_start <-ata_bmdma_qc_issue
kworker/-71      0d....11 13496181us : smp_apic_timer_interrupt <-apic_timer_interrupt
kworker/-71      0d....11 13496181us : irq_enter <-smp_apic_timer_interrupt
kworker/-71      0d....11 13496182us : rcu_irq_enter <-irq_enter

I don't have a lot of experience with this; is this a DMA start for disk I/O, presumably causing an exit from the VM?  For almost 13 milliseconds????

I haven't been able to find much obvious about eliminating this -- any suggestions?  I set caching to writeback without effect.

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writeback'/>
      <source file='/boot.qcow2'/>
      <backingStore type='file' index='1'>
        <format type='raw'/>
        <source file='/root.qcow2'/>
        <backingStore/>
      </backingStore>
      <target dev='hdb' bus='ide'/>
      <alias name='ide0-0-1'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>

Joe Buehler