Re: [RFC PATCH 2/2] arm64, tlbflush: don't TLBI broadcast if page reused in write fault

[Ryan Roberts <ryan.roberts@arm.com>]

On 18/09/2025 03:18, Huang, Ying wrote:
> Hi, Ryan,
> 
> Thanks for reivew!
> 
> Ryan Roberts <ryan.roberts@arm.com> writes:
> 
>> On 15/09/2025 04:29, Huang Ying wrote:
>>> A multi-thread customer workload with large memory footprint uses
>>> fork()/exec() to run some external programs every tens seconds.  When
>>> running the workload on an arm64 server machine, it's observed that
>>> quite some CPU cycles are spent in the TLB flushing functions.  While
>>> running the workload on the x86_64 server machine, it's not.  This
>>> causes the performance on arm64 to be much worse than that on x86_64.
>>>
>>> During the workload running, after fork()/exec() write-protects all
>>> pages in the parent process, memory writing in the parent process
>>> will cause a write protection fault.  Then the page fault handler
>>> will make the PTE/PDE writable if the page can be reused, which is
>>> almost always true in the workload.  On arm64, to avoid the write
>>> protection fault on other CPUs, the page fault handler flushes the TLB
>>> globally with TLBI broadcast after changing the PTE/PDE.  However, this
>>> isn't always necessary.  Firstly, it's safe to leave some stall
>>> read-only TLB entries as long as they will be flushed finally.
>>> Secondly, it's quite possible that the original read-only PTE/PDEs
>>> aren't cached in remote TLB at all if the memory footprint is large.
>>> In fact, on x86_64, the page fault handler doesn't flush the remote
>>> TLB in this situation, which benefits the performance a lot.
>>>
>>> To improve the performance on arm64, make the write protection fault
>>> handler flush the TLB locally instead of globally via TLBI broadcast
>>> after making the PTE/PDE writable.  If there are stall read-only TLB
>>> entries in the remote CPUs, the page fault handler on these CPUs will
>>> regard the page fault as spurious and flush the stall TLB entries.
>>>
>>> To test the patchset, make the usemem.c from
>>> vm-scalability (https://git.kernel.org/pub/scm/linux/kernel/git/wfg/vm-scalability.git).
>>> support calling fork()/exec() periodically.  To mimic the behavior of
>>> the customer workload, run usemem with 4 threads, access 100GB memory,
>>> and call fork()/exec() every 40 seconds.  Test results show that with
>>> the patchset the score of usemem improves ~40.6%.  The cycles% of TLB
>>> flush functions reduces from ~50.5% to ~0.3% in perf profile.
>>
>> Overall, this looks like a simple and useful performance optimization - thanks!
>> I'm running this through our performance regression suite to see if it spots any
>> workloads where the change slows things down and will report once we have the
>> results.
> 
> Thanks!
> 
>>> Signed-off-by: Huang Ying <ying.huang@linux.alibaba.com>
>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>> Cc: Will Deacon <will@kernel.org>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: Yang Shi <yang@os.amperecomputing.com>
>>> Cc: "Christoph Lameter (Ampere)" <cl@gentwo.org>
>>> Cc: Dev Jain <dev.jain@arm.com>
>>> Cc: Barry Song <baohua@kernel.org>
>>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>>> Cc: Yicong Yang <yangyicong@hisilicon.com>
>>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
>>> Cc: Yin Fengwei <fengwei_yin@linux.alibaba.com>
>>> Cc: linux-arm-kernel@lists.infradead.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Cc: linux-mm@kvack.org
>>> ---
>>>  arch/arm64/include/asm/pgtable.h  | 14 ++++++++-----
>>>  arch/arm64/include/asm/tlbflush.h | 33 +++++++++++++++++++++++++++++++
>>>  arch/arm64/mm/fault.c             |  2 +-
>>>  3 files changed, 43 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>> index abd2dee416b3..a9ed8c9d2c33 100644
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -130,12 +130,16 @@ static inline void arch_leave_lazy_mmu_mode(void)
>>>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>>  
>>>  /*
>>> - * Outside of a few very special situations (e.g. hibernation), we always
>>> - * use broadcast TLB invalidation instructions, therefore a spurious page
>>> - * fault on one CPU which has been handled concurrently by another CPU
>>> - * does not need to perform additional invalidation.
>>> + * We use local TLB invalidation instruction when reusing page in
>>> + * write protection fault handler to avoid TLBI broadcast in the hot
>>> + * path.  This will cause spurious page faults if stall read-only TLB
>>> + * entries exist.
>>>   */
>>> -#define flush_tlb_fix_spurious_fault(vma, address, ptep) do { } while (0)
>>> +#define flush_tlb_fix_spurious_fault(vma, address, ptep)	\
>>> +	local_flush_tlb_page_nonotify(vma, address)
>>> +
>>> +#define flush_tlb_fix_spurious_fault_pmd(vma, address, pmdp)	\
>>> +	local_flush_tlb_page_nonotify(vma, address)
>>
>> It's not really clear to me how important doing local tlb flushes for pmds is
>> for the performance improvement? I'm guessing most of the win comes from the pte
>> level? I suspect you have only added spurious pmd fault handling because the
>> arm64 implementation of __ptep_set_access_flags() actually handles both pte and
>> pmd levels?
>>
>> Given the core kernel didn't previously have support for pmd spurious faults, I
>> wonder if it would be simpler to drop the first patch and rejig
>> __ptep_set_access_flags() so that it has a pgsize parameter and can
>> differentiate based on that? I'm on the fence...
>>
>> If you do end up taking this approach, there is a style I introduced for
>> hugetlb, where the function is suffixed with _anysz and it takes a pgsize param:
>>
>> int __ptep_set_access_flags_anysz(struct vm_area_struct *vma,
>> 				  unsigned long address, pte_t *ptep,
>> 				  pte_t entry, int dirty, unsigned long pgsize);
> 
> From the performance point of view, local TLB flushes don't improve
> performance much.  At least in a test similar to the one above, we don't
> find observable improvement.
> 
> From the design point of view, I personally prefer to use similar logic
> between PMD and PTE unless it hurts performance.  IMHO, less different
> logic means less mental overhead.  Do you agree?

Yeah fair enough, that's a reasonable argument.

> 
>>>  
>>>  /*
>>>   * ZERO_PAGE is a global shared page that is always zero: used
>>> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
>>> index 18a5dc0c9a54..607b67d8f61b 100644
>>> --- a/arch/arm64/include/asm/tlbflush.h
>>> +++ b/arch/arm64/include/asm/tlbflush.h
>>> @@ -282,6 +282,39 @@ static inline void flush_tlb_mm(struct mm_struct *mm)
>>>  	mmu_notifier_arch_invalidate_secondary_tlbs(mm, 0, -1UL);
>>>  }
>>>  
>>> +static inline void __local_flush_tlb_page_nonotify_nosync(
>>> +	struct mm_struct *mm, unsigned long uaddr)
>>> +{
>>> +	unsigned long addr;
>>> +
>>> +	dsb(nshst);
>>> +	addr = __TLBI_VADDR(uaddr, ASID(mm));
>>> +	__tlbi(vale1, addr);
>>> +	__tlbi_user(vale1, addr);
>>> +}
>>> +
>>> +static inline void local_flush_tlb_page_nonotify(
>>> +	struct vm_area_struct *vma, unsigned long uaddr)
>>> +{
>>> +	__local_flush_tlb_page_nonotify_nosync(vma->vm_mm, uaddr);
>>> +	dsb(nsh);
>>> +}
>>> +
>>> +static inline void __local_flush_tlb_page_nosync(
>>> +	struct mm_struct *mm, unsigned long uaddr)
>>> +{
>>> +	__local_flush_tlb_page_nonotify_nosync(mm, uaddr);
>>> +	mmu_notifier_arch_invalidate_secondary_tlbs(mm, uaddr & PAGE_MASK,
>>> +						(uaddr & PAGE_MASK) + PAGE_SIZE);
>>> +}
>>> +
>>> +static inline void local_flush_tlb_page(struct vm_area_struct *vma,
>>> +					unsigned long uaddr)
>>> +{
>>> +	__local_flush_tlb_page_nosync(vma->vm_mm, uaddr);
>>> +	dsb(nsh);
>>> +}
>>> +
>>
>> You're introducing more variants than you're actually using here. I think you
>> just need local_flush_tlb_page() and local_flush_tlb_page_nonotify(); you could
>> keep __local_flush_tlb_page_nonotify_nosync() and drop
>> __local_flush_tlb_page_nosync() since it's not really adding much?
> 
> Sure.
> 
>> But I'm also wondering if we should tidy up this API in general; we have local
>> vs broadcast, sync vs nosync, notify vs nonotify. And page is really just a
>> special case of a range. So perhaps it is better to rework the range API to take
>> some flags and we can tidy up all of this. I know Will also posted an RFC to
>> convert a lot of this to c functions, which should also be included. Not a
>> blocker for this change, I don't think, but there should definitely be some
>> follow up work to tidy it up. (I'm happy to take it on).
> 
> This sounds goo to me.  Thanks for working on this!

OK I'll put it on my todo list to have a go at this as follow up.

> 
>>>  static inline void __flush_tlb_page_nosync(struct mm_struct *mm,
>>>  					   unsigned long uaddr)
>>>  {
>>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>>> index d816ff44faff..22f54f5afe3f 100644
>>> --- a/arch/arm64/mm/fault.c
>>> +++ b/arch/arm64/mm/fault.c
>>> @@ -235,7 +235,7 @@ int __ptep_set_access_flags(struct vm_area_struct *vma,
>>>  
>>>  	/* Invalidate a stale read-only entry */
>>>  	if (dirty)
>>> -		flush_tlb_page(vma, address);
>>> +		local_flush_tlb_page(vma, address);
>>
>> Given this is called for both pmds and ptes, it's a bit of an abuse to flush a
>> *page*. Yes it is architecturally safe, but it's not exactly self-documenting.
>> If we pass in the pgsize (as per above) it could be optimized given we know the
>> level and we only want to invalidate the last level. e.g. the local equivalent to:
>>
>> __flush_tlb_range(vma, address, address + PMD_SIZE, PMD_SIZE, true, 2);
>>
>> or
>>
>> __flush_tlb_range(vma, address, address + PAGE_SIZE, PAGE_SIZE, true, 3);
>>
>> Again though, perhaps that is part of some follow up tidying work?
> 
> This sounds good to me too.  And, Yes, I think this can be a follow up
> tidying work.

I'll include this in my follow up.

> 
>> contpte_ptep_set_access_flags() currently does a (broadcast) __flush_tlb_range()
>> on the (pte_write(orig_pte) == pte_write(entry)) path. I think that should be
>> changed to a local range invalidation to be consistent with this change.
> 
> Yes.  This should be changed too.  However, it means we need a local
> variant of __flush_tlb_range() and flush_tlb_mm().  Is it OK to
> introduce them first and tidy up later?

I think do it as Catalin suggested for now. I'll then refactor as part of the
tidy up.

Thanks,
Ryan

> 
>>>  	return 1;
>>>  }
>>>  
> 
> ---
> Best Regards,
> Huang, Ying