xen: sched/Credit2: fix bug when moving CPUs between two Credit2 cpupools

xen: sched/Credit2: fix bug when moving CPUs between two Credit2 cpupools

Whether or not a CPU is assigned to a runqueue (and, if yes, to which
one) within a Credit2 scheduler instance must be both a per-cpu and
per-scheduler instance one.

In fact, when we move a CPU between cpupools, we first setup its per-cpu
data in the new pool, and then cleanup its per-cpu data from the old
pool. In Credit2, when there currently is no per-scheduler, per-cpu
data (as the cpu-to-runqueue map is stored on a per-cpu basis only),
this means that the cleanup of the old per-cpu data can mess with the
new per-cpu data, leading to crashes like this:

https://www.mail-archive.com/xen-devel@lists.xenproject.org/msg23306.html
https://www.mail-archive.com/xen-devel@lists.xenproject.org/msg23350.html

Basically, when csched2_deinit_pdata() is called for CPU 13, for fully
removing the CPU from Pool-0, per_cpu(13,runq_map) already contain the
id of the runqueue to which the CPU has been assigned in the scheduler
of Pool-1, which means wrong runqueue manipulations happen in Pool-0's
scheduler. Furthermore, at the end of such call, that same runq_map is
updated with -1, which is what causes the BUG_ON in csched2_schedule(),
on CPU 13, to trigger.

So, instead of reverting a2c4e5ab59d "xen: credit2: make the cpu to
runqueue map per-cpu" (as we don't want to go back to having the huge
array in struct csched2_private) add a per-cpu scheduler specific data
structure, like, for instance, Credit1 has already. That (for now) only
contains one field: the id of the runqueue the CPU is assigned to.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>