On 17/10/2017 23:34, Andi Kleen wrote:
> One issue I see for some time is that even when I stop a Linux KVM guest using libvirt
> it still sometimes uses a lot of CPU time.
>
> According to strace it loops polling on KVM_IRQ_LINE_STATUS
>
> I see it with various qemu versions, both FC25, FC26, OpenSUSE Leap 42.1/2
>
> IIRC old enough KVM didn't have this problem, at least I didn't notice it.
>
> My workaround is to kill -STOP the qemu process too, but is there a
> better fix?
>
> -Andi
>
> strace of a stopped qemu using ~18% CPU time of a core.
>
> write(7, "\1\0\0\0\0\0\0\0", 8) = 8
> clock_gettime(CLOCK_MONOTONIC, {tv_sec=1118693, tv_nsec=583125997}) = 0
> gettimeofday({tv_sec=1508275611, tv_usec=558425}, NULL) = 0
> clock_gettime(CLOCK_MONOTONIC, {tv_sec=1118693, tv_nsec=583171115}) = 0
> ppoll([{fd=3, events=POLLIN}, {fd=5, events=POLLIN}, {fd=7,
> events=POLLIN}, {fd=8, events=POLLIN}, {fd=13, events=POLLIN}, {fd=15,
> events=POLLIN}, {fd=24, events=POLLIN}], 7, {tv_sec=0, tv_nsec=0}, NULL,
> 8) = 1 ([{fd=7, revents=POLLIN}], left {tv_sec=0, tv_nsec=0})
Because the polling time is zero here, and fd=7 has POLLIN, it seems
that QEMU is continuously scheduling a callback (a timer with a deadline
in the past, or a bottom half). Can you perhaps attach gdb and see what
callback it is?
Paolo
