Re: [PATCH] KVM:PPC Issue in exit timing clearance

[Alexander Graf <agraf@suse.de>]

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