diff mbox series

dynamically allocate struct cpumask

Message ID 20200127071602.11460-1-nick.desaulniers@gmail.com (mailing list archive)
State New, archived
Headers show
Series dynamically allocate struct cpumask | expand

Commit Message

Nick Desaulniers Jan. 27, 2020, 7:16 a.m. UTC 
This helps avoid avoid a potentially large stack allocation.

When building with:
$ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
The following warning is observed:
arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
vector)
            ^
Debugging with:
https://github.com/ClangBuiltLinux/frame-larger-than
via:
$ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
  kvm_send_ipi_mask_allbutself
points to the stack allocated `struct cpumask newmask` in
`kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
8192, making a single instance of a `struct cpumask` 1024 B.

Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
---
 arch/x86/kernel/kvm.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

Comments

Peter Zijlstra Jan. 27, 2020, 8:09 a.m. UTC | #1 
On Sun, Jan 26, 2020 at 11:16:02PM -0800, Nick Desaulniers wrote:
> This helps avoid avoid a potentially large stack allocation.
> 
> When building with:
> $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
> The following warning is observed:
> arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
> function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
> static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
> vector)
>             ^
> Debugging with:
> https://github.com/ClangBuiltLinux/frame-larger-than
> via:
> $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
>   kvm_send_ipi_mask_allbutself
> points to the stack allocated `struct cpumask newmask` in
> `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
> potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
> the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
> 8192, making a single instance of a `struct cpumask` 1024 B.
> 
> Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> ---
>  arch/x86/kernel/kvm.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 32ef1ee733b7..d41c0a0d62a2 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -494,13 +494,15 @@ static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
>  static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
>  {
>  	unsigned int this_cpu = smp_processor_id();
> -	struct cpumask new_mask;

Right, on stack cpumask is definitely dodgy.

> +	struct cpumask *new_mask;
>  	const struct cpumask *local_mask;
>  
> -	cpumask_copy(&new_mask, mask);
> -	cpumask_clear_cpu(this_cpu, &new_mask);
> -	local_mask = &new_mask;
> +	new_mask = kmalloc(sizeof(*new_mask), GFP_KERNEL);
> +	cpumask_copy(new_mask, mask);
> +	cpumask_clear_cpu(this_cpu, new_mask);
> +	local_mask = new_mask;
>  	__send_ipi_mask(local_mask, vector);
> +	kfree(new_mask);
>  }

One alternative approach is adding the inverse of cpu_bit_bitmap. I'm
not entirely sure how often we need the all-but-self mask, but ISTR
there were other places too.
Nick Desaulniers Jan. 27, 2020, 8:14 a.m. UTC | #2 
On Mon, Jan 27, 2020 at 12:09 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Sun, Jan 26, 2020 at 11:16:02PM -0800, Nick Desaulniers wrote:
> > This helps avoid avoid a potentially large stack allocation.
> >
> > When building with:
> > $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
> > The following warning is observed:
> > arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
> > function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
> > static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
> > vector)
> >             ^
> > Debugging with:
> > https://github.com/ClangBuiltLinux/frame-larger-than
> > via:
> > $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
> >   kvm_send_ipi_mask_allbutself
> > points to the stack allocated `struct cpumask newmask` in
> > `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
> > potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
> > the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
> > 8192, making a single instance of a `struct cpumask` 1024 B.
> >
> > Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> > ---
> >  arch/x86/kernel/kvm.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 32ef1ee733b7..d41c0a0d62a2 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -494,13 +494,15 @@ static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
> >  static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
> >  {
> >       unsigned int this_cpu = smp_processor_id();
> > -     struct cpumask new_mask;
>
> Right, on stack cpumask is definitely dodgy.
>
> > +     struct cpumask *new_mask;
> >       const struct cpumask *local_mask;
> >
> > -     cpumask_copy(&new_mask, mask);
> > -     cpumask_clear_cpu(this_cpu, &new_mask);
> > -     local_mask = &new_mask;
> > +     new_mask = kmalloc(sizeof(*new_mask), GFP_KERNEL);

Probably should check for allocation failure, d'oh!

> > +     cpumask_copy(new_mask, mask);
> > +     cpumask_clear_cpu(this_cpu, new_mask);
> > +     local_mask = new_mask;
> >       __send_ipi_mask(local_mask, vector);
> > +     kfree(new_mask);
> >  }
>
> One alternative approach is adding the inverse of cpu_bit_bitmap. I'm
> not entirely sure how often we need the all-but-self mask, but ISTR
> there were other places too.
Vitaly Kuznetsov Jan. 27, 2020, 8:56 a.m. UTC | #3 
Nick Desaulniers <nick.desaulniers@gmail.com> writes:

> This helps avoid avoid a potentially large stack allocation.
>
> When building with:
> $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
> The following warning is observed:
> arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
> function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
> static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
> vector)
>             ^
> Debugging with:
> https://github.com/ClangBuiltLinux/frame-larger-than
> via:
> $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
>   kvm_send_ipi_mask_allbutself
> points to the stack allocated `struct cpumask newmask` in
> `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
> potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
> the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
> 8192, making a single instance of a `struct cpumask` 1024 B.
>
> Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> ---
>  arch/x86/kernel/kvm.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 32ef1ee733b7..d41c0a0d62a2 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -494,13 +494,15 @@ static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
>  static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
>  {
>  	unsigned int this_cpu = smp_processor_id();
> -	struct cpumask new_mask;
> +	struct cpumask *new_mask;
>  	const struct cpumask *local_mask;
>  
> -	cpumask_copy(&new_mask, mask);
> -	cpumask_clear_cpu(this_cpu, &new_mask);
> -	local_mask = &new_mask;
> +	new_mask = kmalloc(sizeof(*new_mask), GFP_KERNEL);

You could've used alloc_cpumask_var() instead, however, I think that
memory allocation on this path is undesireable. We can always
pre-allocate 1 cpumask variable per cpu and use it every time, e.g. we
do this for Hyper-V.

> +	cpumask_copy(new_mask, mask);
> +	cpumask_clear_cpu(this_cpu, new_mask);
> +	local_mask = new_mask;
>  	__send_ipi_mask(local_mask, vector);
> +	kfree(new_mask);
>  }
>  
>  /*
Wanpeng Li Feb. 3, 2020, 8:31 a.m. UTC | #4 
Hi Nick,
On Mon, 27 Jan 2020 at 15:16, Nick Desaulniers
<nick.desaulniers@gmail.com> wrote:
>
> This helps avoid avoid a potentially large stack allocation.
>
> When building with:
> $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
> The following warning is observed:
> arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
> function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
> static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
> vector)
>             ^
> Debugging with:
> https://github.com/ClangBuiltLinux/frame-larger-than
> via:
> $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
>   kvm_send_ipi_mask_allbutself
> points to the stack allocated `struct cpumask newmask` in
> `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
> potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
> the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
> 8192, making a single instance of a `struct cpumask` 1024 B.

Could you help test the below untested patch?

From 867753e2fa27906f15df7902ba1bce7f9cef6ebe Mon Sep 17 00:00:00 2001
From: Wanpeng Li <wanpengli@tencent.com>
Date: Mon, 3 Feb 2020 16:26:35 +0800
Subject: [PATCH] KVM: Pre-allocate 1 cpumask variable per cpu for both
pv tlb and pv ipis

Reported-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kernel/kvm.c | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 81045aab..b1e8efa 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -425,6 +425,8 @@ static void __init sev_map_percpu_data(void)
     }
 }

+static DEFINE_PER_CPU(cpumask_var_t, __pv_cpu_mask);
+
 #ifdef CONFIG_SMP
 #define KVM_IPI_CLUSTER_SIZE    (2 * BITS_PER_LONG)

@@ -490,12 +492,12 @@ static void kvm_send_ipi_mask(const struct
cpumask *mask, int vector)
 static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask,
int vector)
 {
     unsigned int this_cpu = smp_processor_id();
-    struct cpumask new_mask;
+    struct cpumask *new_mask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
     const struct cpumask *local_mask;

-    cpumask_copy(&new_mask, mask);
-    cpumask_clear_cpu(this_cpu, &new_mask);
-    local_mask = &new_mask;
+    cpumask_copy(new_mask, mask);
+    cpumask_clear_cpu(this_cpu, new_mask);
+    local_mask = new_mask;
     __send_ipi_mask(local_mask, vector);
 }

@@ -575,7 +577,6 @@ static void __init kvm_apf_trap_init(void)
     update_intr_gate(X86_TRAP_PF, async_page_fault);
 }

-static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);

 static void kvm_flush_tlb_others(const struct cpumask *cpumask,
             const struct flush_tlb_info *info)
@@ -583,7 +584,7 @@ static void kvm_flush_tlb_others(const struct
cpumask *cpumask,
     u8 state;
     int cpu;
     struct kvm_steal_time *src;
-    struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
+    struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);

     cpumask_copy(flushmask, cpumask);
     /*
@@ -624,6 +625,7 @@ static void __init kvm_guest_init(void)
         kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
         pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
         pv_ops.mmu.tlb_remove_table = tlb_remove_table;
+        pr_info("KVM setup pv remote TLB flush\n");
     }

     if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
@@ -732,23 +734,30 @@ static __init int activate_jump_labels(void)
 }
 arch_initcall(activate_jump_labels);

-static __init int kvm_setup_pv_tlb_flush(void)
+static __init int kvm_alloc_cpumask(void)
 {
     int cpu;
+    bool alloc = false;

     if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
         !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
-        kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
+        kvm_para_has_feature(KVM_FEATURE_STEAL_TIME))
+        alloc = true;
+
+#if defined(CONFIG_SMP)
+    if (!alloc && kvm_para_has_feature(KVM_FEATURE_PV_SEND_IPI))
+        alloc = true;
+#endif
+
+    if (alloc)
         for_each_possible_cpu(cpu) {
-            zalloc_cpumask_var_node(per_cpu_ptr(&__pv_tlb_mask, cpu),
+            zalloc_cpumask_var_node(per_cpu_ptr(&__pv_cpu_mask, cpu),
                 GFP_KERNEL, cpu_to_node(cpu));
         }
-        pr_info("KVM setup pv remote TLB flush\n");
-    }

     return 0;
 }
-arch_initcall(kvm_setup_pv_tlb_flush);
+arch_initcall(kvm_alloc_cpumask);

 #ifdef CONFIG_PARAVIRT_SPINLOCKS

--
1.8.3.1
Nick Desaulniers Feb. 3, 2020, 3:02 p.m. UTC | #5 
On Mon, Feb 3, 2020 at 8:32 AM Wanpeng Li <kernellwp@gmail.com> wrote:
>
> Hi Nick,
> On Mon, 27 Jan 2020 at 15:16, Nick Desaulniers
> <nick.desaulniers@gmail.com> wrote:
> >
> > This helps avoid avoid a potentially large stack allocation.
> >
> > When building with:
> > $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
> > The following warning is observed:
> > arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
> > function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
> > static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
> > vector)
> >             ^
> > Debugging with:
> > https://github.com/ClangBuiltLinux/frame-larger-than
> > via:
> > $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
> >   kvm_send_ipi_mask_allbutself
> > points to the stack allocated `struct cpumask newmask` in
> > `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
> > potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
> > the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
> > 8192, making a single instance of a `struct cpumask` 1024 B.
>
> Could you help test the below untested patch?
>
> From 867753e2fa27906f15df7902ba1bce7f9cef6ebe Mon Sep 17 00:00:00 2001
> From: Wanpeng Li <wanpengli@tencent.com>
> Date: Mon, 3 Feb 2020 16:26:35 +0800
> Subject: [PATCH] KVM: Pre-allocate 1 cpumask variable per cpu for both
> pv tlb and pv ipis
>
> Reported-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kernel/kvm.c | 33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 81045aab..b1e8efa 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -425,6 +425,8 @@ static void __init sev_map_percpu_data(void)
>      }
>  }
>
> +static DEFINE_PER_CPU(cpumask_var_t, __pv_cpu_mask);
> +
>  #ifdef CONFIG_SMP
>  #define KVM_IPI_CLUSTER_SIZE    (2 * BITS_PER_LONG)
>
> @@ -490,12 +492,12 @@ static void kvm_send_ipi_mask(const struct
> cpumask *mask, int vector)
>  static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask,
> int vector)
>  {
>      unsigned int this_cpu = smp_processor_id();
> -    struct cpumask new_mask;
> +    struct cpumask *new_mask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);

Yes, this should help reduce the stack usage, thanks.
Acked-by: Nick Desaulniers <ndesaulniers@google.com>

>      const struct cpumask *local_mask;
>
> -    cpumask_copy(&new_mask, mask);
> -    cpumask_clear_cpu(this_cpu, &new_mask);
> -    local_mask = &new_mask;
> +    cpumask_copy(new_mask, mask);
> +    cpumask_clear_cpu(this_cpu, new_mask);
> +    local_mask = new_mask;
>      __send_ipi_mask(local_mask, vector);
>  }
>
> @@ -575,7 +577,6 @@ static void __init kvm_apf_trap_init(void)
>      update_intr_gate(X86_TRAP_PF, async_page_fault);
>  }
>
> -static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);
>
>  static void kvm_flush_tlb_others(const struct cpumask *cpumask,
>              const struct flush_tlb_info *info)
> @@ -583,7 +584,7 @@ static void kvm_flush_tlb_others(const struct
> cpumask *cpumask,
>      u8 state;
>      int cpu;
>      struct kvm_steal_time *src;
> -    struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
> +    struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
>
>      cpumask_copy(flushmask, cpumask);
>      /*
> @@ -624,6 +625,7 @@ static void __init kvm_guest_init(void)
>          kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
>          pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
>          pv_ops.mmu.tlb_remove_table = tlb_remove_table;
> +        pr_info("KVM setup pv remote TLB flush\n");
>      }
>
>      if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
> @@ -732,23 +734,30 @@ static __init int activate_jump_labels(void)
>  }
>  arch_initcall(activate_jump_labels);
>
> -static __init int kvm_setup_pv_tlb_flush(void)
> +static __init int kvm_alloc_cpumask(void)
>  {
>      int cpu;
> +    bool alloc = false;
>
>      if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
>          !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
> -        kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
> +        kvm_para_has_feature(KVM_FEATURE_STEAL_TIME))
> +        alloc = true;
> +
> +#if defined(CONFIG_SMP)
> +    if (!alloc && kvm_para_has_feature(KVM_FEATURE_PV_SEND_IPI))
> +        alloc = true;
> +#endif
> +
> +    if (alloc)
>          for_each_possible_cpu(cpu) {
> -            zalloc_cpumask_var_node(per_cpu_ptr(&__pv_tlb_mask, cpu),
> +            zalloc_cpumask_var_node(per_cpu_ptr(&__pv_cpu_mask, cpu),
>                  GFP_KERNEL, cpu_to_node(cpu));
>          }
> -        pr_info("KVM setup pv remote TLB flush\n");
> -    }
>
>      return 0;
>  }
> -arch_initcall(kvm_setup_pv_tlb_flush);
> +arch_initcall(kvm_alloc_cpumask);
>
>  #ifdef CONFIG_PARAVIRT_SPINLOCKS
>
> --
> 1.8.3.1
>
> --
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diff mbox series

Patch

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 32ef1ee733b7..d41c0a0d62a2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -494,13 +494,15 @@  static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
 static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
 {
 	unsigned int this_cpu = smp_processor_id();
-	struct cpumask new_mask;
+	struct cpumask *new_mask;
 	const struct cpumask *local_mask;
 
-	cpumask_copy(&new_mask, mask);
-	cpumask_clear_cpu(this_cpu, &new_mask);
-	local_mask = &new_mask;
+	new_mask = kmalloc(sizeof(*new_mask), GFP_KERNEL);
+	cpumask_copy(new_mask, mask);
+	cpumask_clear_cpu(this_cpu, new_mask);
+	local_mask = new_mask;
 	__send_ipi_mask(local_mask, vector);
+	kfree(new_mask);
 }
 
 /*