Re: [PATCH] qemu-kvm: response to SIGUSR1 to start/stop a VCPU (v2)

Anthony Liguori
2010-11-24T13:59:36Z

On 11/24/2010 02:18 AM, Avi Kivity wrote:
> On 11/23/2010 06:49 PM, Anthony Liguori wrote:
>> qemu-kvm vcpu threads don't response to SIGSTOP/SIGCONT. Instead of
>> teaching
>> them to respond to these signals (which cannot be trapped), use
>> SIGUSR1 to
>> approximate the behavior of SIGSTOP/SIGCONT.
>>
>> The purpose of this is to implement CPU hard limits using an external
>> tool that
>> watches the CPU consumption and stops the VCPU as appropriate.
>>
>> This provides a more elegant solution in that it allows the VCPU
>> thread to
>> release qemu_mutex before going to sleep.
>>
>> This current implementation uses a single signal. I think this is
>> too racey
>> in the long term so I think we should introduce a second signal. If
>> two signals
>> get coalesced into one, it could confuse the monitoring tool into
>> giving the
>> VCPU the inverse of it's entitlement.
>
> You can use sigqueue() to send an accompanying value.

I switched to using SIGRTMIN+5 and SIGRTMIN+6. I think that's a nicer
solution since it maps to SIGCONT/SIGSTOP.

>> It might be better to simply move this logic entirely into QEMU to
>> make this
>> more robust--the question is whether we think this is a good long
>> term feature
>> to carry in QEMU?
>>
>
> I'm more concerned about lock holder preemption, and interaction of
> this mechanism with any kernel solution for LHP.

Can you suggest some scenarios and I'll create some test cases? I'm
trying figure out the best way to evaluate this.

Are you assuming the existence of a directed yield and the specific
concern is what happens when a directed yield happens after a PLE and
the target of the yield has been capped?

>> +static __thread int sigusr1_wfd;
>> +
>> +static void on_sigusr1(int signo)
>> +{
>> + char ch = 0;
>> + if (write(sigusr1_wfd,&ch, 1)< 0) {
>> + /* who cares */
>> + }
>> +}
>
> We do have signalfd().

This is actually called from signalfd. I thought about refactoring that
loop to handle signals directly but since we do this elsewhere I figured
I'd keep things consistent.

>> +
>> +static void sigusr1_read(void *opaque)
>> +{
>> + CPUState *env = opaque;
>> + ssize_t len;
>> + int caught_signal = 0;
>> +
>> + do {
>> + char buffer[256];
>> + len = read(env->sigusr1_fd, buffer, sizeof(buffer));
>> + caught_signal = 1;
>> + } while (len> 0);
>> +
>> + if (caught_signal) {
>> + if (env->stopped) {
>
> env->stopped is multiplexed among multiple users, so this interferes
> with vm_stop().
>
> We need to make ->stopped a reference count instead.

Indeed.

Regards,

Anthony Liguori