linux-kernel - [PATCH v5 6/8] x86/mm/tlb: Do not make is

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Message-Id: <20210209221653.614098-7-namit@vmware.com>
Date:   Tue,  9 Feb 2021 14:16:51 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Nadav Amit <namit@...are.com>
Subject: [PATCH v5 6/8] x86/mm/tlb: Do not make is_lazy dirty for no reason

From: Nadav Amit <namit@...are.com>

Blindly writing to is_lazy for no reason, when the written value is
identical to the old value, makes the cacheline dirty for no reason.
Avoid making such writes to prevent cache coherency traffic for no
reason.

Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
Signed-off-by: Nadav Amit <namit@...are.com>
---
 arch/x86/mm/tlb.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index e0271e0f84ea..98d212518f67 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -469,7 +469,8 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		__flush_tlb_all();
 	}
 #endif
-	this_cpu_write(cpu_tlbstate_shared.is_lazy, false);
+	if (was_lazy)
+		this_cpu_write(cpu_tlbstate_shared.is_lazy, false);
 
 	/*
 	 * The membarrier system call requires a full memory barrier and
-- 
2.25.1