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Re: [Qemu-devel] KVM VM(rhel-5.5) %si is too high when TX/RX packets

From: Zhanghaoyu (A)
Subject: Re: [Qemu-devel] KVM VM(rhel-5.5) %si is too high when TX/RX packets
Date: Sat, 4 May 2013 09:25:50 +0000

>> I running a VM(RHEL-5.5) on KVM hypervisor(linux-3.8 + QEMU-1.4.1), 
>> and direct-assign intel 82576 VF to the VM. When TX/RX packets on VM to the 
>> other host via iperf tool, top tool result on VM shown that the %si is too 
>> high, approximately 95% ~ 100%, but from the view of host, the VM's total 
>> CPU usage is about 20% - 30%. And the throughput rate is approximately 
>> 200Mb/s, far from the line rate 1Gb/s, And, I found  the hardirq rate is 
>> lower than normal by running "watch -d -n 1 cat /proc/interrupts", I think 
>> it's caused by the too high %si, because the NIC's hardirq was disabled 
>> during the softirq process.
>> Then, I direct-assign the intel 82576 to the VM, the same case happened too. 
>> I found the intel 82576 and intel 82576 VF's interrupt mode are both 
>> And,
>> I rmmod the igb driver, and, re-insmod the igb driver(igb-4.1.2) with the 
>> parameter IntMode=0/1(0:legacy, 1:MSI, 2:MSI-x), the problem then gone, the 
>> %si is approximately 20% -30%, and the throughput rate came to the line 
>> rate, about 940Mb/s.
>> I update the VM to RHEL-6.1, the problem disappeared too.
>> And, I found a very strange thing, the VM's 82576VF's irq routing is set one 
>> time on Vf's one interrupt received, so frequently.
>RHEL 5.5 is a very old update.  Can you try RHEL 5.9?
>In any case, this looks a lot like a bug in the version of the driver that was 
>included in RHEL5.5; you should contact Red Hat support services if you can 
>still reproduce it with the latest RHEL5 update.

One patch has been proposed to QEMU, shown as below,

[PATCH] [KVM] Needless to update msi route when only msi-x entry "control" 
section changed
With regard to old version linux guest(e.g., rhel-5.5), in ISR processing, mask 
and unmask msi-x vector every time, which result in VMEXIT, then QEMU will 
invoke kvm_irqchip_update_msi_route() to ask KVM hypervisor to update the VM 
irq routing table. In KVM hypervisor, synchronizing RCU needed after updating 
routing table, so much time consumed for waiting in wait_rcu_gp(). So CPU usage 
in VM is so high, while from the view of host, VM's total CPU usage is so low. 
Masking/unmasking msi-x vector only set msi-x entry "control" section, needless 
to update VM irq routing table.

hw/i386/kvm/pci-assign.c | 3 +++
1 files changed, 3 insertions(+)

--- a/hw/i386/kvm/pci-assign.c  2013-05-04 15:53:18.000000000 +0800
+++ b/hw/i386/kvm/pci-assign.c  2013-05-04 15:50:46.000000000 +0800
@@ -1576,6 +1576,8 @@ static void assigned_dev_msix_mmio_write
                 MSIMessage msg;
                 int ret;

+                /* Needless to update msi route when only msi-x entry 
"control" section changed */
+                if ((addr & (PCI_MSIX_ENTRY_SIZE - 1)) != 
                 msg.address = entry->addr_lo |
                     ((uint64_t)entry->addr_hi << 32);
                 msg.data = entry->data; @@ -1585,6 +1587,7 @@ static void 
                 if (ret) {
                     error_report("Error updating irq routing entry (%d)", ret);
+                }

Zhang Haoyu

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