x86/mm/tlb: Put cpumask_test_cpu() check in switch_mm_irqs_off() under CONFIG_DEBUG_VM

On a web server workload, the cpumask_test_cpu() inside the
WARN_ON_ONCE() in the 'prev == next branch' takes about 17% of
all the CPU time of switch_mm_irqs_off().

On a large fleet, this WARN_ON_ONCE() has not fired in at least
a month, possibly never.

Move this test under CONFIG_DEBUG_VM so it does not get compiled
in production kernels.

Signed-off-by: Rik van Riel <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: rikvanriel

arch/x86/mm/tlb.c

@@ -568,7 +568,7 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
 		 * mm_cpumask. The TLB shootdown code can figure out from
 		 * cpu_tlbstate_shared.is_lazy whether or not to send an IPI.
 		 */
-		if (WARN_ON_ONCE(prev != &init_mm &&
+		if (IS_ENABLED(CONFIG_DEBUG_VM) && WARN_ON_ONCE(prev != &init_mm &&
 				 !cpumask_test_cpu(cpu, mm_cpumask(next))))
 			cpumask_set_cpu(cpu, mm_cpumask(next));