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From: Peter Zijlstra <a.p.zijlstra@chello.nl>
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Date: Wed, 01 Dec 2010 18:45:02 +0100
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Subject: [Qemu-devel] Re: [PATCH] qemu-kvm: response to SIGUSR1 to
	start/stop a VCPU (v2)
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To: vatsa@linux.vnet.ibm.com
Cc: Mike@gnu.org, kvm@vger.kernel.org, Galbraith <efault@gmx.de>, qemu-devel@nongnu.org, Chris Wright <chrisw@sous-sol.org>, Anthony Liguori <aliguori@linux.vnet.ibm.com>, Avi Kivity <avi@redhat.com>

On Wed, 2010-12-01 at 22:59 +0530, Srivatsa Vaddagiri wrote:
>=20
> yield_task_fair(...)
> {
>=20
> +       ideal_runtime =3D sched_slice(cfs_rq, curr);
> +       delta_exec =3D curr->sum_exec_runtime - curr->prev_sum_exec_runti=
me;
> +       rem_time_slice =3D ideal_runtime - delta_exec;
> +
> +       current->donate_time +=3D rem_time_slice > some_threshold ?
> +                                some_threshold : rem_time_slice;
>=20
>         ...
> }
>=20
>=20
> sched_slice(...)
> {
>         slice =3D ...
>=20
> +       slice +=3D current->donate_time;
>=20
> }
>=20
> or something close to it. I am bit reluctant to go that route myself, unl=
ess the
> fairness issue with plain yield is quite bad.=20

That really won't do anything. You need to adjust both tasks their
vruntime. Also, I really wouldn't touch the yield() implementation, nor
would I expose any such time donation crap to userspace.