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 <riel@surriel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20241109003727.3958374-4-riel@surriel.com

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 86593d1b787d8a5b9fa4bd492356898ec8870938..b0d5a644fc84dc2c405d42bb102fd20873441744 100644 (file)
--- a/arch/x86/mm/tlb.c
+++ b/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));