Re: [PATCH 7.2 v10 2/2] x86/tlb: skip redundant sync IPIs for native TLB flush

Lance Yang
2026-04-24T15:50:25Z

On Fri, Apr 24, 2026 at 05:12:47PM +0200, Peter Zijlstra wrote:
>On Fri, Apr 24, 2026 at 02:25:28PM +0800, Lance Yang wrote:
>> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
>> index cfcb60468b01..2cf1eeaffd6f 100644
>> --- a/arch/x86/hyperv/mmu.c
>> +++ b/arch/x86/hyperv/mmu.c
>> @@ -63,7 +63,7 @@ static void hyperv_flush_tlb_multi(const struct cpumask *cpus,
>> struct hv_tlb_flush *flush;
>> u64 status;
>> unsigned long flags;
>> - bool do_lazy = !info->freed_tables;
>> + bool do_lazy = !info->wake_lazy_cpus;
>>
>> trace_hyperv_mmu_flush_tlb_multi(cpus, info);
>>
>> @@ -198,7 +198,7 @@ static u64 hyperv_flush_tlb_others_ex(const struct cpumask *cpus,
>>
>> flush->hv_vp_set.format = HV_GENERIC_SET_SPARSE_4K;
>> nr_bank = cpumask_to_vpset_skip(&flush->hv_vp_set, cpus,
>> - info->freed_tables ? NULL : cpu_is_lazy);
>> + info->wake_lazy_cpus ? NULL : cpu_is_lazy);
>> if (nr_bank < 0)
>> return HV_STATUS_INVALID_PARAMETER;
>>
>> diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
>> index 866ea78ba156..fb256fd95f95 100644
>> --- a/arch/x86/include/asm/tlb.h
>> +++ b/arch/x86/include/asm/tlb.h
>
>> static inline void tlb_flush(struct mmu_gather *tlb)
>> {
>> unsigned long start = 0UL, end = TLB_FLUSH_ALL;
>> unsigned int stride_shift = tlb_get_unmap_shift(tlb);
>>
>> + /*
>> + * Both freed_tables and unshared_tables must wake lazy-TLB CPUs, so
>> + * they receive IPIs before reusing or freeing page tables, allowing
>> + * us to safely implement tlb_table_flush_implies_ipi_broadcast().
>> + */
>> + bool wake_lazy_cpus = tlb->freed_tables || tlb->unshared_tables;
>> +
>> if (!tlb->fullmm && !tlb->need_flush_all) {
>> start = tlb->start;
>> end = tlb->end;
>> }
>>
>> - flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables);
>> + flush_tlb_mm_range(tlb->mm, start, end, stride_shift, wake_lazy_cpus);
>> }
>>
>> static inline void invlpg(unsigned long addr)
>> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
>> index 5a3cdc439e38..39b9454781c3 100644
>> --- a/arch/x86/include/asm/tlbflush.h
>> +++ b/arch/x86/include/asm/tlbflush.h
>> @@ -225,7 +227,7 @@ struct flush_tlb_info {
>> u64 new_tlb_gen;
>> unsigned int initiating_cpu;
>> u8 stride_shift;
>> - u8 freed_tables;
>> + u8 wake_lazy_cpus;
>> u8 trim_cpumask;
>> };
>>
>> @@ -315,7 +317,7 @@ static inline bool mm_in_asid_transition(struct mm_struct *mm) { return false; }
>> extern void flush_tlb_all(void);
>> extern void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>> unsigned long end, unsigned int stride_shift,
>> - bool freed_tables);
>> + bool wake_lazy_cpus);
>> extern void flush_tlb_kernel_range(unsigned long start, unsigned long end);
>>
>> static inline void flush_tlb_page(struct vm_area_struct *vma, unsigned long a)
>
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 621e09d049cb..3ce254a3982c 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -1360,16 +1362,16 @@ STATIC_NOPV void native_flush_tlb_multi(const struct cpumask *cpumask,
>> (info->end - info->start) >> PAGE_SHIFT);
>>
>> /*
>> - * If no page tables were freed, we can skip sending IPIs to
>> - * CPUs in lazy TLB mode. They will flush the CPU themselves
>> - * at the next context switch.
>> + * If lazy-TLB CPUs do not need to be woken, we can skip sending
>> + * IPIs to them. They will flush themselves at the next context
>> + * switch.
>> *
>> - * However, if page tables are getting freed, we need to send the
>> - * IPI everywhere, to prevent CPUs in lazy TLB mode from tripping
>> - * up on the new contents of what used to be page tables, while
>> - * doing a speculative memory access.
>> + * However, if page tables are getting freed or unshared, we need
>> + * to send the IPI everywhere, to prevent CPUs in lazy TLB mode
>> + * from tripping up on the new contents of what used to be page
>> + * tables, while doing a speculative memory access.
>> */
>> - if (info->freed_tables || mm_in_asid_transition(info->mm))
>> + if (info->wake_lazy_cpus || mm_in_asid_transition(info->mm))
>> on_each_cpu_mask(cpumask, flush_tlb_func, (void *)info, true);
>> else
>> on_each_cpu_cond_mask(should_flush_tlb, flush_tlb_func,
>> @@ -1402,7 +1404,7 @@ static DEFINE_PER_CPU(unsigned int, flush_tlb_info_idx);
>>
>> static struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm,
>> unsigned long start, unsigned long end,
>> - unsigned int stride_shift, bool freed_tables,
>> + unsigned int stride_shift, bool wake_lazy_cpus,
>> u64 new_tlb_gen)
>> {
>> struct flush_tlb_info *info = this_cpu_ptr(&flush_tlb_info);
>> @@ -1429,7 +1431,7 @@ static struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm,
>> info->end = end;
>> info->mm = mm;
>> info->stride_shift = stride_shift;
>> - info->freed_tables = freed_tables;
>> + info->wake_lazy_cpus = wake_lazy_cpus;
>> info->new_tlb_gen = new_tlb_gen;
>> info->initiating_cpu = smp_processor_id();
>> info->trim_cpumask = 0;
>> @@ -1448,7 +1450,7 @@ static void put_flush_tlb_info(void)
>>
>> void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>> unsigned long end, unsigned int stride_shift,
>> - bool freed_tables)
>> + bool wake_lazy_cpus)
>> {
>> struct flush_tlb_info *info;
>> int cpu = get_cpu();
>> @@ -1457,7 +1459,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>> /* This is also a barrier that synchronizes with switch_mm(). */
>> new_tlb_gen = inc_mm_tlb_gen(mm);
>>
>> - info = get_flush_tlb_info(mm, start, end, stride_shift, freed_tables,
>> + info = get_flush_tlb_info(mm, start, end, stride_shift, wake_lazy_cpus,
>> new_tlb_gen);
>>
>> /*
>
>This whole s/freed_tables/wake_lazy_cpus/ rename should probably be its
>own patch, as should that include unshare_tables thing be.
>
>That seems like unrelated changes.

Thanks, makes sense! Will split the pure s/freed_tables/wake_lazy_cpus/
rename out.

For the tlb->unshared_tables part, I would keep it with this patch,
since lazy-TLB CPUs still have to be woken before reusing unshared page
tables.

@Dave what do you think?

Thanks,
Lance