linux-kernel - [PATCH 3/3] x86,tlb: put cpumask_test_cpu in prev == next under CONFIG_DEBUG

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Message-ID: <20241109003727.3958374-4-riel@surriel.com>
Date: Fri,  8 Nov 2024 19:27:50 -0500
From: Rik van Riel <riel@...riel.com>
To: linux-kernel@...r.kernel.org
Cc: dave.hansen@...ux.intel.com,
	luto@...nel.org,
	peterz@...radead.org,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	x86@...nel.org,
	kernel-team@...a.com,
	hpa@...or.com,
	Rik van Riel <riel@...riel.com>
Subject: [PATCH 3/3] x86,tlb: put cpumask_test_cpu in prev == next 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@...riel.com>
---
 arch/x86/mm/tlb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 9d0d34576928..1aac4fa90d3d 100644
--- 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));
 
-- 
2.45.2