xen/sched: let credit scheduler control its timer all alone

xen/sched: let credit scheduler control its timer all alone

The credit scheduler is the only scheduler with tick_suspend and
tick_resume callbacks. Today those callbacks are invoked without being
guarded by the scheduler lock which is critical when at the same the
cpu those callbacks are active is being moved to or from a cpupool.

Crashes like the following are possible due to that race:

(XEN) ----[ Xen-4.13.0-8.0.12-d  x86_64  debug=y   Not tainted ]----
(XEN) CPU:    79
(XEN) RIP:    e008:[<ffff82d0802467dc>] set_timer+0x39/0x1f7
(XEN) RFLAGS: 0000000000010002   CONTEXT: hypervisor
<snip>
(XEN) Xen call trace:
(XEN)    [<ffff82d0802467dc>] set_timer+0x39/0x1f7
(XEN)    [<ffff82d08022c1f4>]
sched_credit.c#csched_tick_resume+0x54/0x59
(XEN)    [<ffff82d080241dfe>] sched_tick_resume+0x67/0x86
(XEN)    [<ffff82d0802eda52>] mwait-idle.c#mwait_idle+0x32b/0x357
(XEN)    [<ffff82d08027939e>] domain.c#idle_loop+0xa6/0xc2
(XEN)
(XEN) Pagetable walk from 0000000000000048:
(XEN)  L4[0x000] = 00000082cfb9c063 ffffffffffffffff
(XEN)  L3[0x000] = 00000082cfb9b063 ffffffffffffffff
(XEN)  L2[0x000] = 00000082cfb9a063 ffffffffffffffff
(XEN)  L1[0x000] = 0000000000000000 ffffffffffffffff
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 79:
(XEN) FATAL PAGE FAULT
(XEN) [error_code=0000]
(XEN) Faulting linear address: 0000000000000048
(XEN) ****************************************

The callbacks are used when the cpu is going to or coming from idle in
order to allow higher C-states.

The credit scheduler knows when it is going to schedule an idle
scheduling unit or another one after idle, so it can easily stop or
resume the timer itself removing the need to do so via the callback.
As this timer handling is done in the main scheduling function the
scheduler lock is still held, so the race with cpupool operations can
no longer occur. Note that calling the callbacks from schedule_cpu_rm()
and schedule_cpu_add() is no longer needed, as the transitions to and
from idle in the cpupool with credit active will automatically occur
and do the right thing.

With the last user of the callbacks gone those can be removed.

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