Re: [PATCH] dynamically allocate struct cpumask

[Nick Desaulniers]

On Mon, Feb 3, 2020 at 8:32 AM Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
> Hi Nick,
> On Mon, 27 Jan 2020 at 15:16, Nick Desaulniers
> <nick.desaulniers@xxxxxxxxx> 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@xxxxxxxxxxx>
> 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@xxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
>  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@xxxxxxxxxx>

>      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
>
> --
> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxxx
> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CANRm%2BCwK0Cg45mktda9Yz9fsjPCvtuB8O%2Bfma5L3tV725ki1qw%40mail.gmail.com.



-- 
Thanks,
~Nick Desaulniers