sched/core: Optimize in_task() and in_interrupt() a bit

sched/core: Optimize in_task() and in_interrupt() a bit

Except on x86, preempt_count is always accessed with READ_ONCE().
Repeated invocations in macros like irq_count() produce repeated loads.
These redundant instructions appear in various fast paths. In the one
shown below, for example, irq_count() is evaluated during kernel entry
if !tick_nohz_full_cpu(smp_processor_id()).

0001ed0a <irq_enter_rcu>:
   1ed0a:       4e56 0000       linkw %fp,#0
   1ed0e:       200f            movel %sp,%d0
   1ed10:       0280 ffff e000  andil #-8192,%d0
   1ed16:       2040            moveal %d0,%a0
   1ed18:       2028 0008       movel %a0@(8),%d0
   1ed1c:       0680 0001 0000  addil #65536,%d0
   1ed22:       2140 0008       movel %d0,%a0@(8)
   1ed26:       082a 0001 000f  btst #1,%a2@(15)
   1ed2c:       670c            beqs 1ed3a <irq_enter_rcu+0x30>
   1ed2e:       2028 0008       movel %a0@(8),%d0
   1ed32:       2028 0008       movel %a0@(8),%d0
   1ed36:       2028 0008       movel %a0@(8),%d0
   1ed3a:       4e5e            unlk %fp
   1ed3c:       4e75            rts

This patch doesn't prevent the pointless btst and beqs instructions
above, but it does eliminate 2 of the 3 pointless move instructions
here and elsewhere.

On x86, preempt_count is per-cpu data and the problem does not arise
presumably because the compiler is free to optimize more effectively.

This patch was tested on m68k and x86. I was expecting no changes
to object code for x86 and mostly that's what I saw. However, there
were a few places where code generation was perturbed for some reason.

The performance issue addressed here is minor on uniprocessor m68k. I
got a 0.01% improvement from this patch for a simple "find /sys -false"
benchmark. For architectures and workloads susceptible to cache line bounce
the improvement is expected to be larger. The only SMP architecture I have
is x86, and as x86 unaffected I have not done any further measurements.

Fixes: 15115830c887 ("preempt: Cleanup the macro maze a bit")
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/0a403120a682a525e6db2d81d1a3ffcc137c3742.1694756831.git.fthain@linux-m68k.org