From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH] qemu-kvm: response to SIGUSR1 to start/stop a VCPU (v2)
Date: Wed, 24 Nov 2010 10:18:01 +0200
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Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org,
	Chris Wright <chrisw@sous-sol.org>
To: Anthony Liguori <aliguori@us.ibm.com>
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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.

> 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.

> +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().

> +
> +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.

-- 
error compiling committee.c: too many arguments to function