When switching the hrtimer cpu_base, we briefly allow for
preemption to become enabled by unlocking the cpu_base lock.
During this time, the CPU corresponding to the new cpu_base
that was selected may in fact go offline. In this scenario, the
hrtimer is enqueued to a CPU that's not online, and therefore
it never fires.
As an example, consider this example:
CPU #0 CPU #1
---- ----
... hrtimer_start()
lock_hrtimer_base()
switch_hrtimer_base()
cpu = hrtimer_get_target() -> 1
spin_unlock(&cpu_base->lock)
<migrate thread to CPU #0>
<offline>
spin_lock(&new_base->lock)
this_cpu = 0
cpu != this_cpu
enqueue_hrtimer(cpu_base #1)
To prevent this scenario, verify that the CPU corresponding to
the new cpu_base is indeed online before selecting it in
hrtimer_switch_base(). If it's not online, fallback to using the
base of the current CPU.
Change-Id: I6359cb299ba7e08abf25d3360fcdd925b4c03b69
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
Michael Bohan
878f1bd9dd