Too many rescheduling interrupts (still!)

[Andy Lutomirski]

Rumor has it that Linux 3.13 was supposed to get rid of all the silly
rescheduling interrupts.  It doesn't, although it does seem to have
improved the situation.

A small number of reschedule interrupts appear to be due to a race:
both resched_task and wake_up_idle_cpu do, essentially:

set_tsk_need_resched(t);
smb_mb();
if (!tsk_is_polling(t))
  smp_send_reschedule(cpu);

The problem is that set_tsk_need_resched wakes the CPU and, if the CPU
is too quick (which isn't surprising if it was in C0 or C1), then it
could *clear* TS_POLLING before tsk_is_polling is read.

Is there a good reason that TIF_NEED_RESCHED is in thread->flags and
TS_POLLING is in thread->status?  Couldn't both of these be in the
same field in something like struct rq?  That would allow a real
atomic op here.

The more serious issue is that AFAICS default_wake_function is
completely missing the polling check.  It goes through
ttwu_queue_remote, which unconditionally sends an interrupt.

--Andy
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