On 03/22/2011 01:43 PM, Avi Kivity wrote:
On 03/16/2011 06:37 PM, Bharat Bhushan wrote:
Following dump is observed on host when clearing the exit timing
counters
[root@p1021mds kvm]# echo -n 'c'> vm1200_vcpu0_timing
INFO: task echo:1276 blocked for more than 120 seconds.
"echo 0> /proc/sys/kernel/hung_task_timeout_secs" disables this
message.
echo D 0ff5bf94 0 1276 1190 0x00000000
Call Trace:
[c2157e40] [c0007908] __switch_to+0x9c/0xc4
[c2157e50] [c040293c] schedule+0x1b4/0x3bc
[c2157e90] [c04032dc] __mutex_lock_slowpath+0x74/0xc0
[c2157ec0] [c00369e4] kvmppc_init_timing_stats+0x20/0xb8
[c2157ed0] [c0036b00] kvmppc_exit_timing_write+0x84/0x98
[c2157ef0] [c00b9f90] vfs_write+0xc0/0x16c
[c2157f10] [c00ba284] sys_write+0x4c/0x90
[c2157f40] [c000e320] ret_from_syscall+0x0/0x3c
The vcpu->mutex is used by kvm_ioctl_* (KVM_RUN etc) and same was
used when clearing the stats (in kvmppc_init_timing_stats()).
What happens is that when the guest is idle then it held the
vcpu->mutx. While the exiting timing process waits for guest to
release the vcpu->mutex and a hang state is reached.
Now using seprate lock for exit timing stats.
Seems excessive to have a new lock just for timing.
The whole thing is only used for debugging, so being excessive doesn't
hurt too much here. In normal configurations, it's #ifdef'ed out.
What about using vcpu->requests to have the statistics cleared in vcpu
context?
What about dropping the whole thing and replacing it with tracepoints?
That should work. The question is if it's worth the effort. The current
code is there after all :).
Alex
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