From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id B2BDC2D9ECD for ; Thu, 23 Oct 2025 10:54:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761216886; cv=none; b=S8vlFcTTs7n0U3BOStqTmUoh66oFX/FDKfdf0FAq/IJ+gKjuX3PV9m8Tfgf8NJ0DVwaldbSzDiVhu7UetSjDOmmKkVko97VT5lIhaXVHcKWpwk9x/EJDTSMi9A4wXgHxgxI+L+JI0sGjV10sEg+gKFJ/Cn9zSB5ZPUxXPlEy99c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761216886; c=relaxed/simple; bh=NkcPDzz6PRUjrMF/BwtUvta0I0A69Hkq2wRbQUHTNM4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UyUfxjh5ypE41IfX64lz31oII5T3/a4QdlG40PVfdXLr1R9lqssfjzrtYfG37Bez+xOG19vQen58WdEUJBgmK9uKHi7EQSS1TpSwLktEH03B50sQ0omx40Z/EYBOFNZdTv4HJyFbV1yEdv1d2pMBYgFEVIx8bz8p2SV3srrJmvI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CA4891516; Thu, 23 Oct 2025 03:54:34 -0700 (PDT) Received: from [10.1.31.176] (XHFQ2J9959.cambridge.arm.com [10.1.31.176]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C02393F63F; Thu, 23 Oct 2025 03:54:39 -0700 (PDT) Message-ID: Date: Thu, 23 Oct 2025 11:54:38 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH -v3 2/2] arm64, tlbflush: don't TLBI broadcast if page reused in write fault Content-Language: en-GB To: Huang Ying , Catalin Marinas , Will Deacon , Andrew Morton , David Hildenbrand Cc: Lorenzo Stoakes , Vlastimil Babka , Zi Yan , Baolin Wang , Yang Shi , "Christoph Lameter (Ampere)" , Dev Jain , Barry Song , Anshuman Khandual , Kefeng Wang , Kevin Brodsky , Yin Fengwei , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20251023013524.100517-1-ying.huang@linux.alibaba.com> <20251023013524.100517-3-ying.huang@linux.alibaba.com> From: Ryan Roberts In-Reply-To: <20251023013524.100517-3-ying.huang@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 23/10/2025 02:35, 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 stale > 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 stale 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 stale 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. LGTM: Reviewed-by: Ryan Roberts > > Signed-off-by: Huang Ying > Cc: Catalin Marinas > Cc: Will Deacon > Cc: Andrew Morton > Cc: David Hildenbrand > Cc: Lorenzo Stoakes > Cc: Vlastimil Babka > Cc: Zi Yan > Cc: Baolin Wang > Cc: Ryan Roberts > Cc: Yang Shi > Cc: "Christoph Lameter (Ampere)" > Cc: Dev Jain > Cc: Barry Song > Cc: Anshuman Khandual > Cc: Kefeng Wang > Cc: Kevin Brodsky > Cc: Yin Fengwei > 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 | 56 +++++++++++++++++++++++++++++++ > arch/arm64/mm/contpte.c | 3 +- > arch/arm64/mm/fault.c | 2 +- > 4 files changed, 67 insertions(+), 8 deletions(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index aa89c2e67ebc..25b3c31edb6c 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 stale 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) > > /* > * 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..5c8f88fa5e40 100644 > --- a/arch/arm64/include/asm/tlbflush.h > +++ b/arch/arm64/include/asm/tlbflush.h > @@ -249,6 +249,19 @@ static inline unsigned long get_trans_granule(void) > * cannot be easily determined, the value TLBI_TTL_UNKNOWN will > * perform a non-hinted invalidation. > * > + * local_flush_tlb_page(vma, addr) > + * Local variant of flush_tlb_page(). Stale TLB entries may > + * remain in remote CPUs. > + * > + * local_flush_tlb_page_nonotify(vma, addr) > + * Same as local_flush_tlb_page() except MMU notifier will not be > + * called. > + * > + * local_flush_tlb_contpte(vma, addr) > + * Invalidate the virtual-address range > + * '[addr, addr+CONT_PTE_SIZE)' mapped with contpte on local CPU > + * for the user address space corresponding to 'vma->mm'. Stale > + * TLB entries may remain in remote CPUs. > * > * Finally, take a look at asm/tlb.h to see how tlb_flush() is implemented > * on top of these routines, since that is our interface to the mmu_gather > @@ -282,6 +295,33 @@ 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(struct vm_area_struct *vma, > + unsigned long uaddr) > +{ > + __local_flush_tlb_page_nonotify_nosync(vma->vm_mm, uaddr); > + mmu_notifier_arch_invalidate_secondary_tlbs(vma->vm_mm, uaddr & PAGE_MASK, > + (uaddr & PAGE_MASK) + PAGE_SIZE); > + dsb(nsh); > +} > + > static inline void __flush_tlb_page_nosync(struct mm_struct *mm, > unsigned long uaddr) > { > @@ -472,6 +512,22 @@ static inline void __flush_tlb_range(struct vm_area_struct *vma, > dsb(ish); > } > > +static inline void local_flush_tlb_contpte(struct vm_area_struct *vma, > + unsigned long addr) > +{ > + unsigned long asid; > + > + addr = round_down(addr, CONT_PTE_SIZE); > + > + dsb(nshst); > + asid = ASID(vma->vm_mm); > + __flush_tlb_range_op(vale1, addr, CONT_PTES, PAGE_SIZE, asid, > + 3, true, lpa2_is_enabled()); > + mmu_notifier_arch_invalidate_secondary_tlbs(vma->vm_mm, addr, > + addr + CONT_PTE_SIZE); > + dsb(nsh); > +} > + > static inline void flush_tlb_range(struct vm_area_struct *vma, > unsigned long start, unsigned long end) > { > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index c0557945939c..589bcf878938 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -622,8 +622,7 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma, > __ptep_set_access_flags(vma, addr, ptep, entry, 0); > > if (dirty) > - __flush_tlb_range(vma, start_addr, addr, > - PAGE_SIZE, true, 3); > + local_flush_tlb_contpte(vma, start_addr); > } else { > __contpte_try_unfold(vma->vm_mm, addr, ptep, orig_pte); > __ptep_set_access_flags(vma, addr, ptep, entry, dirty); > 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); > return 1; > } >