[Qemu-devel] Lock contention in QEMU

[Weiwei Jia]

Hi Stefan,

I find the timeslice of vCPU thread in QEMU/KVM is unstable when there
are lots of read requests (for example, read 4KB each time (8GB in
total) from one file) from Guest OS. I also find that this phenomenon
may be caused by lock contention in QEMU layer. I find this problem
under following workload.

Workload settings:
In VMM, there are 6 pCPUs which are pCPU0, pCPU1, pCPU2, pCPU3, pCPU4,
pCPU5. There are two Kernel Virtual Machines (VM1 and VM2) upon VMM.
In each VM, there are 5 vritual CPUs (vCPU0, vCPU1, vCPU2, vCPU3,
vCPU4). vCPU0 in VM1 and vCPU0 in VM2 are pinned to pCPU0 and pCPU5
separately to handle interrupts dedicatedly. vCPU1 in VM1 and vCPU1 in
VM2 are pinned to pCPU1; vCPU2 in VM1 and vCPU2 in VM2 are pinned to
pCPU2; vCPU3 in VM1 and vCPU3 in VM2 are pinned to pCPU3; vCPU4 in VM1
and vCPU4 in VM2 are pinned to pCPU4. Besides vCPU0 in VM2 (pinned to
pCPU5), other vCPUs all have one CPU intensive thread (while(1){i++})
upon each of them in VM1 and VM2 to avoid the vCPU to be idle. In VM1,
I start one I/O thread on vCPU2, which the I/O thread reads 4KB from
one file each time (reads 8GB in total). The I/O scheduler in VM1 and
VM2 is NOOP. The I/O scheduler in VMM is CFQ. I also pinned the I/O
worker threads launched by QEMU to pCPU5 (note: there is no CPU
intensive thread on pCPU5 so the I/O requests will be handled by QEMU
I/O thread workers ASAP). The process scheduling class in VM and VMM
is CFS.

Linux Kernel version for VMM is: 3.16.39
Linux Kernel version for VM1 and VM2 is: 4.7.4
QEMU emulator version is: 2.0.0

When I test above workload, I find the timeslice of vCPU2 thread
jitters very much. I suspect this is triggered by lock contention in
QEMU layer since my debug log in front of VMM Linux Kernel's
schedule->__schedule->context_switch is like following. Once the
timeslice jitters very much, following debug information will appear.

7097537 Dec 13 11:22:33 mobius04 kernel: [39163.015789] Call Trace:
7097538 Dec 13 11:22:33 mobius04 kernel: [39163.015791]
[<ffffffff8176b2f0>] dump_stack+0x64/0x84
7097539 Dec 13 11:22:33 mobius04 kernel: [39163.015793]
[<ffffffff8176bf85>] __schedule+0x5b5/0x960
7097540 Dec 13 11:22:33 mobius04 kernel: [39163.015794]
[<ffffffff8176c409>] schedule+0x29/0x70
7097541 Dec 13 11:22:33 mobius04 kernel: [39163.015796]
[<ffffffff810ef4d8>] futex_wait_queue_me+0xd8/0x150
7097542 Dec 13 11:22:33 mobius04 kernel: [39163.015798]
[<ffffffff810ef6fb>] futex_wait+0x1ab/0x2b0
7097543 Dec 13 11:22:33 mobius04 kernel: [39163.015800]
[<ffffffff810eef00>] ? get_futex_key+0x2d0/0x2e0
7097544 Dec 13 11:22:33 mobius04 kernel: [39163.015804]
[<ffffffffc0290105>] ? __vmx_load_host_state+0x125/0x170 [kv
m_intel]
7097545 Dec 13 11:22:33 mobius04 kernel: [39163.015805]
[<ffffffff810f1275>] do_futex+0xf5/0xd20
7097546 Dec 13 11:22:33 mobius04 kernel: [39163.015813]
[<ffffffffc0222690>] ? kvm_vcpu_ioctl+0x100/0x560 [kvm]
7097547 Dec 13 11:22:33 mobius04 kernel: [39163.015816]
[<ffffffff810b06f0>] ? __dequeue_entity+0x30/0x50
7097548 Dec 13 11:22:33 mobius04 kernel: [39163.015818]
[<ffffffff81013d06>] ? __switch_to+0x596/0x690
7097549 Dec 13 11:22:33 mobius04 kernel: [39163.015820]
[<ffffffff811f9f23>] ? do_vfs_ioctl+0x93/0x520
7097550 Dec 13 11:22:33 mobius04 kernel: [39163.015822]
[<ffffffff810f1f1d>] SyS_futex+0x7d/0x170
7097551 Dec 13 11:22:33 mobius04 kernel: [39163.015824]
[<ffffffff8116d1b2>] ? fire_user_return_notifiers+0x42/0x50
7097552 Dec 13 11:22:33 mobius04 kernel: [39163.015826]
[<ffffffff810154b5>] ? do_notify_resume+0xc5/0x100
7097553 Dec 13 11:22:33 mobius04 kernel: [39163.015828]
[<ffffffff81770a8d>] system_call_fastpath+0x1a/0x1f


If true, I think this may be a scalability problem caused by QEMU I/O
part. Do we have a feature in QEMU to avoid this? Would you please
give me some suggestions about how to make the timeslice of vCPU2
thread stable even though there are lots of I/O Read requests on it.
Thank you.

Best,
Weiwei Jia