Re: [RFC PATCH v2 08/12] x86/tlb: Privatize cpu_tlbstate

From: Andy Lutomirski
Date: Fri May 31 2019 - 14:52:15 EST


> On May 30, 2019, at 11:36 PM, Nadav Amit <namit@xxxxxxxxxx> wrote:
>
> cpu_tlbstate is mostly private and only the variable is_lazy is shared.
> This causes some false-sharing when TLB flushes are performed.
>
> Break cpu_tlbstate intro cpu_tlbstate and cpu_tlbstate_shared, and mark
> each one accordingly.
>

At some point weâll want to do a better job with our PV flush code, and I suspect weâll end up with more of this shared again.

> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> ---
> arch/x86/include/asm/tlbflush.h | 40 ++++++++++++++++++---------------
> arch/x86/mm/init.c | 2 +-
> arch/x86/mm/tlb.c | 15 ++++++++-----
> 3 files changed, 32 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index 79272938cf79..a1fea36d5292 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -178,23 +178,6 @@ struct tlb_state {
> u16 loaded_mm_asid;
> u16 next_asid;
>
> - /*
> - * We can be in one of several states:
> - *
> - * - Actively using an mm. Our CPU's bit will be set in
> - * mm_cpumask(loaded_mm) and is_lazy == false;
> - *
> - * - Not using a real mm. loaded_mm == &init_mm. Our CPU's bit
> - * will not be set in mm_cpumask(&init_mm) and is_lazy == false.
> - *
> - * - Lazily using a real mm. loaded_mm != &init_mm, our bit
> - * is set in mm_cpumask(loaded_mm), but is_lazy == true.
> - * We're heuristically guessing that the CR3 load we
> - * skipped more than makes up for the overhead added by
> - * lazy mode.
> - */
> - bool is_lazy;
> -
> /*
> * If set we changed the page tables in such a way that we
> * needed an invalidation of all contexts (aka. PCIDs / ASIDs).
> @@ -240,7 +223,27 @@ struct tlb_state {
> */
> struct tlb_context ctxs[TLB_NR_DYN_ASIDS];
> };
> -DECLARE_PER_CPU_SHARED_ALIGNED(struct tlb_state, cpu_tlbstate);
> +DECLARE_PER_CPU_ALIGNED(struct tlb_state, cpu_tlbstate);
> +
> +struct tlb_state_shared {
> + /*
> + * We can be in one of several states:
> + *
> + * - Actively using an mm. Our CPU's bit will be set in
> + * mm_cpumask(loaded_mm) and is_lazy == false;
> + *
> + * - Not using a real mm. loaded_mm == &init_mm. Our CPU's bit
> + * will not be set in mm_cpumask(&init_mm) and is_lazy == false.
> + *
> + * - Lazily using a real mm. loaded_mm != &init_mm, our bit
> + * is set in mm_cpumask(loaded_mm), but is_lazy == true.
> + * We're heuristically guessing that the CR3 load we
> + * skipped more than makes up for the overhead added by
> + * lazy mode.
> + */
> + bool is_lazy;
> +};
> +DECLARE_PER_CPU_SHARED_ALIGNED(struct tlb_state_shared, cpu_tlbstate_shared);
>
> /*
> * Blindly accessing user memory from NMI context can be dangerous
> @@ -439,6 +442,7 @@ static inline void __native_flush_tlb_one_user(unsigned long addr)
> {
> u32 loaded_mm_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
>
> + //invpcid_flush_one(kern_pcid(loaded_mm_asid), addr);
> asm volatile("invlpg (%0)" ::"r" (addr) : "memory");
>
> if (!static_cpu_has(X86_FEATURE_PTI))
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index fd10d91a6115..34027f36a944 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -951,7 +951,7 @@ void __init zone_sizes_init(void)
> free_area_init_nodes(max_zone_pfns);
> }
>
> -__visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tlb_state, cpu_tlbstate) = {
> +__visible DEFINE_PER_CPU_ALIGNED(struct tlb_state, cpu_tlbstate) = {
> .loaded_mm = &init_mm,
> .next_asid = 1,
> .cr4 = ~0UL, /* fail hard if we screw up cr4 shadow initialization */
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index b0c3065aad5d..755b2bb3e5b6 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -144,7 +144,7 @@ void leave_mm(int cpu)
> return;
>
> /* Warn if we're not lazy. */
> - WARN_ON(!this_cpu_read(cpu_tlbstate.is_lazy));
> + WARN_ON(!this_cpu_read(cpu_tlbstate_shared.is_lazy));
>
> switch_mm(NULL, &init_mm, NULL);
> }
> @@ -276,7 +276,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> {
> struct mm_struct *real_prev = this_cpu_read(cpu_tlbstate.loaded_mm);
> u16 prev_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
> - bool was_lazy = this_cpu_read(cpu_tlbstate.is_lazy);
> + bool was_lazy = this_cpu_read(cpu_tlbstate_shared.is_lazy);
> unsigned cpu = smp_processor_id();
> u64 next_tlb_gen;
> bool need_flush;
> @@ -321,7 +321,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> __flush_tlb_all();
> }
> #endif
> - this_cpu_write(cpu_tlbstate.is_lazy, false);
> + this_cpu_write(cpu_tlbstate_shared.is_lazy, false);
>
> /*
> * The membarrier system call requires a full memory barrier and
> @@ -462,7 +462,7 @@ void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
> if (this_cpu_read(cpu_tlbstate.loaded_mm) == &init_mm)
> return;
>
> - this_cpu_write(cpu_tlbstate.is_lazy, true);
> + this_cpu_write(cpu_tlbstate_shared.is_lazy, true);
> }
>
> /*
> @@ -543,7 +543,7 @@ static void flush_tlb_func_common(const struct flush_tlb_info *f,
> VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].ctx_id) !=
> loaded_mm->context.ctx_id);
>
> - if (this_cpu_read(cpu_tlbstate.is_lazy)) {
> + if (this_cpu_read(cpu_tlbstate_shared.is_lazy)) {
> /*
> * We're in lazy mode. We need to at least flush our
> * paging-structure cache to avoid speculatively reading
> @@ -659,11 +659,14 @@ static void flush_tlb_func_remote(void *info)
>
> static inline bool tlb_is_not_lazy(int cpu)
> {
> - return !per_cpu(cpu_tlbstate.is_lazy, cpu);
> + return !per_cpu(cpu_tlbstate_shared.is_lazy, cpu);
> }
>
> static DEFINE_PER_CPU(cpumask_t, flush_tlb_mask);
>
> +DEFINE_PER_CPU_ALIGNED(struct tlb_state_shared, cpu_tlbstate_shared);
> +EXPORT_PER_CPU_SYMBOL(cpu_tlbstate_shared);
> +
> void native_flush_tlb_multi(const struct cpumask *cpumask,
> const struct flush_tlb_info *info)
> {
> --
> 2.20.1
>