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[PATCH v4 08/12] arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu_mode()

Kevin Brodsky posted 12 patches 1 month, 2 weeks ago
There is a newer version of this series
[PATCH v4 08/12] arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu_mode()
Posted by Kevin Brodsky 1 month, 2 weeks ago 
The generic lazy_mmu layer now tracks whether a task is in lazy MMU
mode. As a result we no longer need a TIF flag for that purpose -
let's use the new in_lazy_mmu_mode() helper instead.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 arch/arm64/include/asm/pgtable.h     | 16 +++-------------
 arch/arm64/include/asm/thread_info.h |  3 +--
 2 files changed, 4 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 535435248923..61ca88f94551 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -62,30 +62,21 @@ static inline void emit_pte_barriers(void)
 
 static inline void queue_pte_barriers(void)
 {
-	unsigned long flags;
-
 	if (in_interrupt()) {
 		emit_pte_barriers();
 		return;
 	}
 
-	flags = read_thread_flags();
-
-	if (flags & BIT(TIF_LAZY_MMU)) {
-		/* Avoid the atomic op if already set. */
-		if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
-			set_thread_flag(TIF_LAZY_MMU_PENDING);
-	} else {
+	if (in_lazy_mmu_mode())
+		test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);
+	else
 		emit_pte_barriers();
-	}
 }
 
 static inline void arch_enter_lazy_mmu_mode(void)
 {
 	if (in_interrupt())
 		return;
-
-	set_thread_flag(TIF_LAZY_MMU);
 }
 
 static inline void arch_flush_lazy_mmu_mode(void)
@@ -103,7 +94,6 @@ static inline void arch_leave_lazy_mmu_mode(void)
 		return;
 
 	arch_flush_lazy_mmu_mode();
-	clear_thread_flag(TIF_LAZY_MMU);
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index f241b8601ebd..4ff8da0767d9 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -84,8 +84,7 @@ void arch_setup_new_exec(void);
 #define TIF_SME_VL_INHERIT	28	/* Inherit SME vl_onexec across exec */
 #define TIF_KERNEL_FPSTATE	29	/* Task is in a kernel mode FPSIMD section */
 #define TIF_TSC_SIGSEGV		30	/* SIGSEGV on counter-timer access */
-#define TIF_LAZY_MMU		31	/* Task in lazy mmu mode */
-#define TIF_LAZY_MMU_PENDING	32	/* Ops pending for lazy mmu mode exit */
+#define TIF_LAZY_MMU_PENDING	31	/* Ops pending for lazy mmu mode exit */
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
-- 
2.47.0
Re: [PATCH v4 08/12] arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu_mode()
Posted by Ryan Roberts 1 month, 1 week ago 
On 29/10/2025 10:09, Kevin Brodsky wrote:
> The generic lazy_mmu layer now tracks whether a task is in lazy MMU
> mode. As a result we no longer need a TIF flag for that purpose -
> let's use the new in_lazy_mmu_mode() helper instead.
> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
> ---
>  arch/arm64/include/asm/pgtable.h     | 16 +++-------------
>  arch/arm64/include/asm/thread_info.h |  3 +--
>  2 files changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 535435248923..61ca88f94551 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -62,30 +62,21 @@ static inline void emit_pte_barriers(void)
>  
>  static inline void queue_pte_barriers(void)
>  {
> -	unsigned long flags;
> -
>  	if (in_interrupt()) {
>  		emit_pte_barriers();
>  		return;
>  	}
>  
> -	flags = read_thread_flags();
> -
> -	if (flags & BIT(TIF_LAZY_MMU)) {
> -		/* Avoid the atomic op if already set. */
> -		if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
> -			set_thread_flag(TIF_LAZY_MMU_PENDING);
> -	} else {
> +	if (in_lazy_mmu_mode())
> +		test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);

This removes the optimization to only do the atomic set operation if the bit is
not already set. I think that should remain.

> +	else
>  		emit_pte_barriers();
> -	}
>  }
>  
>  static inline void arch_enter_lazy_mmu_mode(void)
>  {
>  	if (in_interrupt())
>  		return;

Why are you keeping this test? Surely it can go?

> -
> -	set_thread_flag(TIF_LAZY_MMU);
>  }
>  
>  static inline void arch_flush_lazy_mmu_mode(void)
> @@ -103,7 +94,6 @@ static inline void arch_leave_lazy_mmu_mode(void)
>  		return;
>  
>  	arch_flush_lazy_mmu_mode();
> -	clear_thread_flag(TIF_LAZY_MMU);
>  }
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index f241b8601ebd..4ff8da0767d9 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -84,8 +84,7 @@ void arch_setup_new_exec(void);
>  #define TIF_SME_VL_INHERIT	28	/* Inherit SME vl_onexec across exec */
>  #define TIF_KERNEL_FPSTATE	29	/* Task is in a kernel mode FPSIMD section */
>  #define TIF_TSC_SIGSEGV		30	/* SIGSEGV on counter-timer access */
> -#define TIF_LAZY_MMU		31	/* Task in lazy mmu mode */
> -#define TIF_LAZY_MMU_PENDING	32	/* Ops pending for lazy mmu mode exit */
> +#define TIF_LAZY_MMU_PENDING	31	/* Ops pending for lazy mmu mode exit */
>  
>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
>  #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
Re: [PATCH v4 08/12] arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu_mode()
Posted by David Hildenbrand 1 month, 1 week ago 
On 29.10.25 11:09, Kevin Brodsky wrote:
> The generic lazy_mmu layer now tracks whether a task is in lazy MMU
> mode. As a result we no longer need a TIF flag for that purpose -
> let's use the new in_lazy_mmu_mode() helper instead.
> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
> ---
>   arch/arm64/include/asm/pgtable.h     | 16 +++-------------
>   arch/arm64/include/asm/thread_info.h |  3 +--
>   2 files changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 535435248923..61ca88f94551 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -62,30 +62,21 @@ static inline void emit_pte_barriers(void)
>   
>   static inline void queue_pte_barriers(void)
>   {
> -	unsigned long flags;
> -
>   	if (in_interrupt()) {
>   		emit_pte_barriers();
>   		return;
>   	}
>   
> -	flags = read_thread_flags();
> -
> -	if (flags & BIT(TIF_LAZY_MMU)) {
> -		/* Avoid the atomic op if already set. */
> -		if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
> -			set_thread_flag(TIF_LAZY_MMU_PENDING);
> -	} else {
> +	if (in_lazy_mmu_mode())
> +		test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);

You likely don't want a test_and_set here, which would do a 
test_and_set_bit() -- an atomic rmw.

You only want to avoid the atomic write if already set.

So keep the current

	/* Avoid the atomic op if already set. */
	if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
		set_thread_flag(TIF_LAZY_MMU_PENDING);

-- 
Cheers

David
Re: [PATCH v4 08/12] arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu_mode()
Posted by Kevin Brodsky 1 month, 1 week ago 
On 03/11/2025 16:03, David Hildenbrand wrote:
> On 29.10.25 11:09, Kevin Brodsky wrote:
>> The generic lazy_mmu layer now tracks whether a task is in lazy MMU
>> mode. As a result we no longer need a TIF flag for that purpose -
>> let's use the new in_lazy_mmu_mode() helper instead.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
>> ---
>>   arch/arm64/include/asm/pgtable.h     | 16 +++-------------
>>   arch/arm64/include/asm/thread_info.h |  3 +--
>>   2 files changed, 4 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h
>> b/arch/arm64/include/asm/pgtable.h
>> index 535435248923..61ca88f94551 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -62,30 +62,21 @@ static inline void emit_pte_barriers(void)
>>     static inline void queue_pte_barriers(void)
>>   {
>> -    unsigned long flags;
>> -
>>       if (in_interrupt()) {
>>           emit_pte_barriers();
>>           return;
>>       }
>>   -    flags = read_thread_flags();
>> -
>> -    if (flags & BIT(TIF_LAZY_MMU)) {
>> -        /* Avoid the atomic op if already set. */
>> -        if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
>> -            set_thread_flag(TIF_LAZY_MMU_PENDING);
>> -    } else {
>> +    if (in_lazy_mmu_mode())
>> +        test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);
>
> You likely don't want a test_and_set here, which would do a
> test_and_set_bit() -- an atomic rmw.

Ah yes good point, the new version would do an atomic RMW in all cases.
Simpler code but also slower :/

>
> You only want to avoid the atomic write if already set.
>
> So keep the current
>
>     /* Avoid the atomic op if already set. */
>     if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
>         set_thread_flag(TIF_LAZY_MMU_PENDING); 

Pretty much, since we're now only considering one flag we can simplify
it to:

if (!test_thread_flag(TIF_LAZY_MMU_PENDING))
    set_thread_flag(TIF_LAZY_MMU_PENDING);

- Kevin

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