arm64: Mark __switch_to() as __sched

Unlike most architectures (and only in keeping with powerpc), arm64 has
a non __sched() function on the path to our cpu_switch_to() assembly
function.

It is expected that for a blocked task, in_sched_functions() can be used
to skip all functions between the raw context switch assembly and the
scheduler functions that call into __switch_to(). This is the behaviour
expected by stack_trace_consume_entry_nosched(), and the behaviour we'd
like to have such that we an simplify arm64's __get_wchan()
implementation to use arch_stack_walk().

This patch mark's arm64's __switch_to as __sched. This *will not* change
the behaviour of arm64's current __get_wchan() implementation, which
always performs an initial unwind step which skips __switch_to(). This
*will* change the behaviour of stack_trace_consume_entry_nosched() and
stack_trace_save_tsk() to match their expected behaviour on blocked
tasks, skipping all scheduler-internal functions including
__switch_to().

Other than the above, there should be no functional change as a result
of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211129142849.3056714-4-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/process.c

@@ -490,7 +490,8 @@ void update_sctlr_el1(u64 sctlr)
 /*
  * Thread switching.
  */
-__notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
+__notrace_funcgraph __sched
+struct task_struct *__switch_to(struct task_struct *prev,
 				struct task_struct *next)
 {
 	struct task_struct *last;