From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] qemu-kvm: response to SIGUSR1 to start/stop a VCPU (v2)
Date: Wed, 01 Dec 2010 18:45:02 +0100
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Cc: Avi Kivity <avi@redhat.com>,
	Anthony Liguori <aliguori@linux.vnet.ibm.com>,
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	Chris Wright <chrisw@sous-sol.org>,
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On Wed, 2010-12-01 at 22:59 +0530, Srivatsa Vaddagiri wrote:
> 
> yield_task_fair(...)
> {
> 
> +       ideal_runtime = sched_slice(cfs_rq, curr);
> +       delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> +       rem_time_slice = ideal_runtime - delta_exec;
> +
> +       current->donate_time += rem_time_slice > some_threshold ?
> +                                some_threshold : rem_time_slice;
> 
>         ...
> }
> 
> 
> sched_slice(...)
> {
>         slice = ...
> 
> +       slice += current->donate_time;
> 
> }
> 
> or something close to it. I am bit reluctant to go that route myself, unless the
> fairness issue with plain yield is quite bad. 

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.