xen: sched_credit: improve picking up the idle CPU for a VCPU

xen: sched_credit: improve picking up the idle CPU for a VCPU

In _csched_cpu_pick() we try to select the best possible CPU for
running a VCPU, considering the characteristics of the underlying
hardware (i.e., how many threads, core, sockets, and how busy they
are). What we want is "the idle execution vehicle with the most
idling neighbours in its grouping".

In order to achieve it, we select a CPU from the VCPU's affinity,
giving preference to its current processor if possible, as the basis
for the comparison with all the other CPUs. Problem is, to discount
the VCPU itself when computing this "idleness" (in an attempt to be
fair wrt its current processor), we arbitrarily and unconditionally
consider that selected CPU as idle, even when it is not the case,
for instance:
 1. If the CPU is not the one where the VCPU is running (perhaps due
    to the affinity being changed);
 2. The CPU is where the VCPU is running, but it has other VCPUs in
    its runq, so it won't go idle even if the VCPU in question goes.

This is exemplified in the trace below:

]  3.466115364 x|------|------| d10v1   22005(2:2:5) 3 [ a 1 8 ]
   ... ... ...
   3.466122856 x|------|------| d10v1 runstate_change d10v1
   running->offline
   3.466123046 x|------|------| d?v? runstate_change d32767v0
   runnable->running
   ... ... ...
]  3.466126887 x|------|------| d32767v0   28004(2:8:4) 3 [ a 1 8 ]

22005(...) line (the first line) means _csched_cpu_pick() was called
on VCPU 1 of domain 10, while it is running on CPU 0, and it choose
CPU 8, which is busy ('|'), even if there are plenty of idle
CPUs. That is because, as a consequence of changing the VCPU affinity,
CPU 8 was chosen as the basis for the comparison, and therefore
considered idle (its bit gets unconditionally set in the bitmask
representing the idle CPUs). 28004(...) line means the VCPU is woken
up and queued on CPU 8's runq, where it waits for a context switch or
a migration, in order to be able to execute.

This change fixes things by only considering the "guessed" CPU idle if
the VCPU in question is both running there and is its only runnable
VCPU.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>