what's the "futex" thread doing when create a kvm guest ?

[Hu Jun]

thank you for reminding me.  /proc/$PID/stack shows the function call stack in kernel space,
but yesterday I thought it shows stack in user space.

I only want to understand what's the thread doing , not for a specific issue.

On 02/23/2015 06:17 AM, drago01 wrote:
> On Sun, Feb 22, 2015 at 1:36 PM, Hu Jun <jhu_com at 163.com> wrote:
>> Hi guys:
>>
>> env:
>> [root at cloudf ~]# uname -a
>> Linux cloudf.fedora.com 3.18.7-100.fc20.x86_64 #1 SMP Wed Feb 11 19:01:50
>> UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
>> [root at cloudf ~]# rpm -q qemu-kvm
>> qemu-kvm-1.6.2-13.fc20.x86_64
>>
>> I found there is a "futex" thead after creating a kvm guest by the
>> command:"/usr/bin/qemu-system-x86_64 -machine accel=kvm -hda
>> /opt/todaygoods_SLES_11_SP3_JeOS.x86_64-0.0.2.raw -vnc 192.168.0.110:6"
>>
>> root at cloudf task]# ps aux | grep qemu
>> root     13111  0.3  2.8 546760 227340 pts/3   S+   18:26   0:12 gdb
>> qemu-system-x86_64
>> root     14379 85.2  2.0 699004 165496 pts/3   tl   18:49  31:12
>> /usr/bin/qemu-system-x86_64 -machine accel=kvm -hda
>> /opt/todaygoods_SLES_11_SP3_JeOS.x86_64-0.0.2.raw -vnc 192.168.0.110:6
>>
>> the process 14379 has three threads, I'm familiar with the two of them
>> except the third thread (lwp :14418)
>>
>> [root at cloudf 14379]# cd /proc/14379/
>> [root at cloudf 14379]# cat task/14
>> 14379/ 14416/ 14418/
>> [root at cloudf 14379]# cat task/14379/stack
>> [<ffffffff8121d1e9>] poll_schedule_timeout+0x49/0x70
>> [<ffffffff8121e8e4>] do_sys_poll+0x484/0x5c0
>> [<ffffffff8121eb24>] SyS_poll+0x74/0x110
>> [<ffffffff8174cfa9>] system_call_fastpath+0x12/0x17
>> [<ffffffffffffffff>] 0xffffffffffffffff
>> [root at cloudf 14379]# cat task/14416/stack
>> [<ffffffffa05cf92d>] kvm_arch_vcpu_ioctl_run+0xb0d/0x1280 [kvm]
>> [<ffffffffa05b7bb2>] kvm_vcpu_ioctl+0x292/0x5c0 [kvm]
>> [<ffffffff8121c710>] do_vfs_ioctl+0x2e0/0x4a0
>> [<ffffffff8121c951>] SyS_ioctl+0x81/0xa0
>> [<ffffffff8174cfa9>] system_call_fastpath+0x12/0x17
>> [<ffffffffffffffff>] 0xffffffffffffffff
>> [root at cloudf 14379]# cat task/14418/stack
>> [<ffffffff8110b6e0>] futex_wait_queue_me+0xd0/0x130
>> [<ffffffff8110b9e6>] futex_wait+0x176/0x270
>> [<ffffffff8110dc7e>] do_futex+0x10e/0xac0
>> [<ffffffff8110e6b0>] SyS_futex+0x80/0x180
>> [<ffffffff8174cfa9>] system_call_fastpath+0x12/0x17
>> [<ffffffffffffffff>] 0xffffffffffffffff
>>
>> what's the "futex" thread doing  when create a kvm guest ?  I deeped into
>> the qemu code , but I failed to find where call "futex" system call.
> It is unlikely that you will find any call to "futex" in the source
> code of an application. It doesn't even have glibc interface.
> Most of the time it is an implementation detail for something else.
> For instance pthread_join is implemented using the futex system call.
>
> But the question here is ... what is the problem that you are trying to solve?
>