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Groups > linux.kernel > #1542877

[patch 3/3] x86/process: Optimize TIF_NOTSC switch

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Provide and use a toggle helper instead of doing it with a branch.

x86_64:
3662       8505      16   12183    2f97 Before
3646	   8505	     16	  12167	   2f87	After 

i386:
5906	   9388	   1804	  17098	   42ca	Before
5834	   9324	   1740	  16898	   4202	After

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/include/asm/tlbflush.h |   10 ++++++++++
 arch/x86/kernel/process.c       |   22 ++++------------------
 2 files changed, 14 insertions(+), 18 deletions(-)

--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -110,6 +110,16 @@ static inline void cr4_clear_bits(unsign
 	}
 }
 
+static inline void cr4_toggle_bits(unsigned long mask)
+{
+	unsigned long cr4;
+
+	cr4 = this_cpu_read(cpu_tlbstate.cr4);
+	cr4 ^= mask;
+	this_cpu_write(cpu_tlbstate.cr4, cr4);
+	__write_cr4(cr4);
+}
+
 /* Read the CR4 shadow. */
 static inline unsigned long cr4_read_shadow(void)
 {
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -116,11 +116,6 @@ void flush_thread(void)
 	fpu__clear(&tsk->thread.fpu);
 }
 
-static void hard_disable_TSC(void)
-{
-	cr4_set_bits(X86_CR4_TSD);
-}
-
 void disable_TSC(void)
 {
 	preempt_disable();
@@ -129,15 +124,10 @@ void disable_TSC(void)
 		 * Must flip the CPU state synchronously with
 		 * TIF_NOTSC in the current running context.
 		 */
-		hard_disable_TSC();
+		cr4_set_bits(X86_CR4_TSD);
 	preempt_enable();
 }
 
-static void hard_enable_TSC(void)
-{
-	cr4_clear_bits(X86_CR4_TSD);
-}
-
 static void enable_TSC(void)
 {
 	preempt_disable();
@@ -146,7 +136,7 @@ static void enable_TSC(void)
 		 * Must flip the CPU state synchronously with
 		 * TIF_NOTSC in the current running context.
 		 */
-		hard_enable_TSC();
+		cr4_clear_bits(X86_CR4_TSD);
 	preempt_enable();
 }
 
@@ -212,12 +202,8 @@ void __switch_to_xtra(struct task_struct
 	if ((tifp ^ tifn) & _TIF_BLOCKSTEP)
 		toggle_debugctlmsr(DEBUGCTLMSR_BTF);
 
-	if ((tifp ^ tifn) & _TIF_NOTSC) {
-		if (tifn & _TIF_NOTSC)
-			hard_disable_TSC();
-		else
-			hard_enable_TSC();
-	}
+	if ((tifp ^ tifn) & _TIF_NOTSC)
+		cr4_toggle_bits(X86_CR4_TSD);
 }
 
 /*

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[patch 3/3] x86/process: Optimize TIF_NOTSC switch Thomas Gleixner <tglx@linutronix.de> - 2016-12-15 17:50 +0100
  Re: [patch 3/3] x86/process: Optimize TIF_NOTSC switch Andy Lutomirski <luto@amacapital.net> - 2016-12-15 18:40 +0100
    Re: [patch 3/3] x86/process: Optimize TIF_NOTSC switch Thomas Gleixner <tglx@linutronix.de> - 2016-12-16 10:00 +0100
      Re: [patch 3/3] x86/process: Optimize TIF_NOTSC switch Andy Lutomirski <luto@amacapital.net> - 2016-12-16 19:40 +0100

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