linux-kernel - [PATCH v3 6/9] x86/mm/tlb: Do not make is

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Message-Id: <20190719005837.4150-7-namit@vmware.com>
Date:   Thu, 18 Jul 2019 17:58:34 -0700
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Nadav Amit <namit@...are.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: [PATCH v3 6/9] x86/mm/tlb: Do not make is_lazy dirty for no reason

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@...el.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 af80c274c88d..89f83ad19507 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -322,7 +322,8 @@ 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
-- 
2.20.1