Re: [PATCH 3/7] xen: sched: better handle (not) inserting idle vCPUs in runqueues

[George Dunlap <george.dunlap@citrix.com>]

On 08/10/15 13:52, Dario Faggioli wrote:
> Idle vCPUs should never really be explicitly inserted
> in any of the schedulers' runqueue. In fact, they are
> just put in execution immediately (either on boot, or
> when a pCPU is assigned to a cpupool), and it will be
> the first scheduling decision that --by preempting
> them-- will 'park' them in the queue.
> 
> In fact, avoiding inserting those vCPUs in runqueues
> is something that Credit2 and RTDS intercept and
> forbid already (when implementing insert_vcpu()).
> Credit1 does that implicitly, as the idle vCPU will
> always be already running when insert_vcpu() is called,
> and hence not put in the runqueue.
> 
> Let's make it *always* explicit, as that simplifies
> things by quite a bit. For instance, we can now
> introduce some BUG_ON() guards, with the twofold
> objective of making this assumption clear, and of
> catching misuse and bugs.
> 
> The check for whether we'd be inserting an idle vCPU
> in a queue, now, happens, once and for all schedulers,
> in generic code, at vCPU initialization time, while
> we can just avoid trying (and always failing!) doing
> so when doing cpupools manipulations.
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>

Although it might be easier to evaluate the previous patch if this one
were moved behind it in the series:

Acked-by: George Dunlap <george.dunlap@citrix.com>

 -George