Re: [PATCH] riscv: Add support for BATCHED_UNMAP_TLB_FLUSH

Jisheng Zhang
2024-01-03T11:10:57Z

On Tue, Jan 02, 2024 at 03:18:51PM +0100, Alexandre Ghiti wrote:
> Allow to defer the flushing of the TLB when unmapping pges, which allows
> to reduce the numbers of IPI and the number of sfence.vma.
>
> The ubenchmarch used in commit 43b3dfdd0455 ("arm64: support
> batched/deferred tlb shootdown during page reclamation/migration") shows
> good performance improvement and perf reports an important decrease in
> time spent flushing the tlb (results come from qemu):

Hi Alex,

I tried this micro benchmark with your patch on T-HEAD TH1520 platform, I
didn't see any performance improvement for the micro benchmark. Per
myunderstanding, the micro benchmark is special case for arm64 because
in a normal tlb flush flow, below sequence is necessary:

tlbi
dsb

while with BATCHED_UNMAP_TLB_FLUSH, the arm64 just does 'tlbi', leaving
the dsb to the arch_tlbbatch_flush(). So the final result is

several 'tlbi + dsb' sequence VS. several 'tlbi' instructions + only one dsb
The performance improvement comes from the unnecessary dsb eliminations.

Do you have suitable benchmark(s) for BATCHED_UNMAP_TLB_FLUSH on riscv?

Thanks

>
> Before this patch:
>
> real 2m1.135s
> user 0m0.980s
> sys 2m0.096s
>
> 4.83% batch_tlb [kernel.kallsyms] [k] __flush_tlb_range
>
> After this patch:
>
> real 1m0.543s
> user 0m1.059s
> sys 0m59.489s
>
> 0.14% batch_tlb [kernel.kallsyms] [k] __flush_tlb_range
>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> arch/riscv/Kconfig | 1 +
> arch/riscv/include/asm/tlbbatch.h | 15 +++++++
> arch/riscv/include/asm/tlbflush.h | 10 +++++
> arch/riscv/mm/tlbflush.c | 71 ++++++++++++++++++++++---------
> 4 files changed, 77 insertions(+), 20 deletions(-)
> create mode 100644 arch/riscv/include/asm/tlbbatch.h
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 7603bd8ab333..aa07bd43b138 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -53,6 +53,7 @@ config RISCV
> select ARCH_USE_MEMTEST
> select ARCH_USE_QUEUED_RWLOCKS
> select ARCH_USES_CFI_TRAPS if CFI_CLANG
> + select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH if SMP && MMU
> select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> select ARCH_WANT_FRAME_POINTERS
> select ARCH_WANT_GENERAL_HUGETLB if !RISCV_ISA_SVNAPOT
> diff --git a/arch/riscv/include/asm/tlbbatch.h b/arch/riscv/include/asm/tlbbatch.h
> new file mode 100644
> index 000000000000..46014f70b9da
> --- /dev/null
> +++ b/arch/riscv/include/asm/tlbbatch.h
> @@ -0,0 +1,15 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2023 Rivos Inc.
> + */
> +
> +#ifndef _ASM_RISCV_TLBBATCH_H
> +#define _ASM_RISCV_TLBBATCH_H
> +
> +#include <linux/cpumask.h>
> +
> +struct arch_tlbflush_unmap_batch {
> + struct cpumask cpumask;
> +};
> +
> +#endif /* _ASM_RISCV_TLBBATCH_H */
> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> index 8f3418c5f172..f0b731ccc0c2 100644
> --- a/arch/riscv/include/asm/tlbflush.h
> +++ b/arch/riscv/include/asm/tlbflush.h
> @@ -46,6 +46,16 @@ void flush_tlb_kernel_range(unsigned long start, unsigned long end);
> void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
> unsigned long end);
> #endif
> +
> +#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> +bool arch_tlbbatch_should_defer(struct mm_struct *mm);
> +void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
> + struct mm_struct *mm,
> + unsigned long uaddr);
> +void arch_flush_tlb_batched_pending(struct mm_struct *mm);
> +void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
> +#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
> +
> #else /* CONFIG_SMP && CONFIG_MMU */
>
> #define flush_tlb_all() local_flush_tlb_all()
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index e6659d7368b3..bb623bca0a7d 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -93,29 +93,23 @@ static void __ipi_flush_tlb_range_asid(void *info)
> local_flush_tlb_range_asid(d->start, d->size, d->stride, d->asid);
> }
>
> -static void __flush_tlb_range(struct mm_struct *mm, unsigned long start,
> - unsigned long size, unsigned long stride)
> +static void __flush_tlb_range(struct cpumask *cmask, unsigned long asid,
> + unsigned long start, unsigned long size,
> + unsigned long stride)
> {
> struct flush_tlb_range_data ftd;
> - const struct cpumask *cmask;
> - unsigned long asid = FLUSH_TLB_NO_ASID;
> bool broadcast;
>
> - if (mm) {
> - unsigned int cpuid;
> + if (cpumask_empty(cmask))
> + return;
>
> - cmask = mm_cpumask(mm);
> - if (cpumask_empty(cmask))
> - return;
> + if (cmask != cpu_online_mask) {
> + unsigned int cpuid;
>
> cpuid = get_cpu();
> /* check if the tlbflush needs to be sent to other CPUs */
> broadcast = cpumask_any_but(cmask, cpuid) < nr_cpu_ids;
> -
> - if (static_branch_unlikely(&use_asid_allocator))
> - asid = atomic_long_read(&mm->context.id) & asid_mask;
> } else {
> - cmask = cpu_online_mask;
> broadcast = true;
> }
>
> @@ -135,25 +129,34 @@ static void __flush_tlb_range(struct mm_struct *mm, unsigned long start,
> local_flush_tlb_range_asid(start, size, stride, asid);
> }
>
> - if (mm)
> + if (cmask != cpu_online_mask)
> put_cpu();
> }
>
> +static inline unsigned long get_mm_asid(struct mm_struct *mm)
> +{
> + return static_branch_unlikely(&use_asid_allocator) ?
> + atomic_long_read(&mm->context.id) & asid_mask : FLUSH_TLB_NO_ASID;
> +}
> +
> void flush_tlb_mm(struct mm_struct *mm)
> {
> - __flush_tlb_range(mm, 0, FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
> + __flush_tlb_range(mm_cpumask(mm), get_mm_asid(mm),
> + 0, FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
> }
>
> void flush_tlb_mm_range(struct mm_struct *mm,
> unsigned long start, unsigned long end,
> unsigned int page_size)
> {
> - __flush_tlb_range(mm, start, end - start, page_size);
> + __flush_tlb_range(mm_cpumask(mm), get_mm_asid(mm),
> + start, end - start, page_size);
> }
>
> void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
> {
> - __flush_tlb_range(vma->vm_mm, addr, PAGE_SIZE, PAGE_SIZE);
> + __flush_tlb_range(mm_cpumask(vma->vm_mm), get_mm_asid(vma->vm_mm),
> + addr, PAGE_SIZE, PAGE_SIZE);
> }
>
> void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
> @@ -185,18 +188,46 @@ void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
> }
> }
>
> - __flush_tlb_range(vma->vm_mm, start, end - start, stride_size);
> + __flush_tlb_range(mm_cpumask(vma->vm_mm), get_mm_asid(vma->vm_mm),
> + start, end - start, stride_size);
> }
>
> void flush_tlb_kernel_range(unsigned long start, unsigned long end)
> {
> - __flush_tlb_range(NULL, start, end - start, PAGE_SIZE);
> + __flush_tlb_range((struct cpumask *)cpu_online_mask, FLUSH_TLB_NO_ASID,
> + start, end - start, PAGE_SIZE);
> }
>
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
> unsigned long end)
> {
> - __flush_tlb_range(vma->vm_mm, start, end - start, PMD_SIZE);
> + __flush_tlb_range(mm_cpumask(vma->vm_mm), get_mm_asid(vma->vm_mm),
> + start, end - start, PMD_SIZE);
> }
> #endif
> +
> +#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> +bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> +{
> + return true;
> +}
> +
> +void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
> + struct mm_struct *mm,
> + unsigned long uaddr)
> +{
> + cpumask_or(&batch->cpumask, &batch->cpumask, mm_cpumask(mm));
> +}
> +
> +void arch_flush_tlb_batched_pending(struct mm_struct *mm)
> +{
> + flush_tlb_mm(mm);
> +}
> +
> +void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
> +{
> + __flush_tlb_range(&batch->cpumask, FLUSH_TLB_NO_ASID, 0,
> + FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
> +}
> +#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
> --
> 2.39.2
>
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv