Re: [PATCH V4 2/2] riscv: Use use_asid_allocator flush TLB

palmerdabbelt
2021-05-29T23:43:03Z

On Thu, 27 May 2021 00:09:03 PDT (-0700), Christoph Hellwig wrote:
> On Wed, May 26, 2021 at 05:49:21AM +0000, guoren@kernel.org wrote:
>> From: Guo Ren <guoren@linux.alibaba.com>
>>
>> Use static_branch_unlikely(&use_asid_allocator) to keep the origin
>> tlb flush style, so it's no effect on the existing machine. Here
>> are the optimized functions:
>> - flush_tlb_mm
>> - flush_tlb_page
>> - flush_tlb_range
>>
>> All above are based on the below new implement functions:
>> - __sbi_tlb_flush_range_asid
>> - local_flush_tlb_range_asid
>>
>> These functions are based on ASID instead of previous non-ASID
>> tlb_flush implementation which invalidates more useful tlb
>> entries.
>
> I still think the commit message is incomplete and rather misleading.
> Here is what I'd come up with from reading the patch:
>
> ---------
> Subject: add ASID-based tlbflushing methods
>
> Implement optimized version of the tlb flushing routines for systems
> using ASIDs. These are behind the use_asid_allocator static branch to
> not affect existing systems not using ASIDs.
> ---------

That seems much better, thanks.

>> +static inline void local_flush_tlb_range_asid(unsigned long start,
>> + unsigned long size, unsigned long asid)
>> +{
>> + unsigned long tmp, end = ALIGN(start + size, PAGE_SIZE);
>> +
>> + for (tmp = start & PAGE_MASK; tmp < end; tmp += PAGE_SIZE) {
>> + __asm__ __volatile__ ("sfence.vma %0, %1"
>> + :
>> + : "r" (tmp), "r" (asid)
>> + : "memory");
>> + tmp += PAGE_SIZE;
>> + }
>
> This double increments tmp.
>
> Also the non-ASID code switches to a global flush once flushing more
> than a single page. It might be worth documenting the tradeoff in the
> code.

For some reason I thought we'd written this down in the commentary of
teh ISA manual as the suggested huersitic here, but I can't find it so
maybe I'm wrong. If it's actually there it would be good to point that
out, otherwise some documentation seems fine as it'll probably become a
tunable at some point anyway.

>> +static void __sbi_tlb_flush_range_asid(struct cpumask *cmask,
>> + unsigned long start,
>> + unsigned long size,
>> + unsigned long asid)
>> +{
>
> I don't think the calling conventions here are optimal. I'd pass
> the mm_struct as the first argument, as we can derive both the cpumask
> and asid from it instead of doing that in the callers.
>
> But more importantly I think the static branch check can be moved deeper
> into the code to avoid a lot of duplication. What do you think of this
> version?
>
> diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
> index b0659413a080..7030837adc1a 100644
> --- a/arch/riscv/include/asm/mmu_context.h
> +++ b/arch/riscv/include/asm/mmu_context.h
> @@ -33,6 +33,8 @@ static inline int init_new_context(struct task_struct *tsk,
> return 0;
> }
>
> +DECLARE_STATIC_KEY_FALSE(use_asid_allocator);
> +
> #include <asm-generic/mmu_context.h>
>
> #endif /* _ASM_RISCV_MMU_CONTEXT_H */
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 68aa312fc352..45c1b04b105d 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -18,7 +18,7 @@
>
> #ifdef CONFIG_MMU
>
> -static DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
> +DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
>
> static unsigned long asid_bits;
> static unsigned long num_asids;
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index 720b443c4528..d8afbb1269d5 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -4,6 +4,33 @@
> #include <linux/smp.h>
> #include <linux/sched.h>
> #include <asm/sbi.h>
> +#include <asm/mmu_context.h>
> +
> +static inline void local_flush_tlb_all_asid(unsigned long asid)
> +{
> + __asm__ __volatile__ ("sfence.vma x0, %0"
> + :
> + : "r" (asid)
> + : "memory");
> +}
> +
> +static inline void local_flush_tlb_page_asid(unsigned long addr,
> + unsigned long asid)
> +{
> + __asm__ __volatile__ ("sfence.vma %0, %1"
> + :
> + : "r" (addr), "r" (asid)
> + : "memory");
> +}
> +
> +static inline void local_flush_tlb_range_asid(unsigned long start,
> + unsigned long size, unsigned long asid)
> +{
> + unsigned long addr, end = ALIGN(start + size, PAGE_SIZE);
> +
> + for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE)
> + local_flush_tlb_page_asid(addr, asid);
> +}
>
> void flush_tlb_all(void)
> {
> @@ -12,28 +39,43 @@ void flush_tlb_all(void)
>
> /*
> * This function must not be called with cmask being null.
> - * Kernel may panic if cmask is NULL.
> */
> -static void __sbi_tlb_flush_range(struct cpumask *cmask, unsigned long start,
> +static void __sbi_tlb_flush_range(struct mm_struct *mm, unsigned long start,
> unsigned long size)
> {
> + struct cpumask *cmask = mm_cpumask(mm);
> struct cpumask hmask;
> unsigned int cpuid;
> + bool broadcast;
>
> if (cpumask_empty(cmask))
> return;
>
> 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)) {
> + unsigned long asid = atomic_long_read(&mm->context.id);
>
> - if (cpumask_any_but(cmask, cpuid) >= nr_cpu_ids) {
> - /* local cpu is the only cpu present in cpumask */
> - if (size <= PAGE_SIZE)
> + if (broadcast) {
> + riscv_cpuid_to_hartid_mask(cmask, &hmask);
> + sbi_remote_sfence_vma_asid(cpumask_bits(&hmask),
> + start, size, asid);
> + } else if (size != -1) {
> + local_flush_tlb_range_asid(start, size, asid);
> + } else {
> + local_flush_tlb_all_asid(asid);
> + }
> + } else {
> + if (broadcast) {
> + riscv_cpuid_to_hartid_mask(cmask, &hmask);
> + sbi_remote_sfence_vma(cpumask_bits(&hmask),
> + start, size);
> + } else if (size <= PAGE_SIZE) {
> local_flush_tlb_page(start);
> - else
> + } else {
> local_flush_tlb_all();
> - } else {
> - riscv_cpuid_to_hartid_mask(cmask, &hmask);
> - sbi_remote_sfence_vma(cpumask_bits(&hmask), start, size);
> + }
> }
>
> put_cpu();
> @@ -41,16 +83,16 @@ static void __sbi_tlb_flush_range(struct cpumask *cmask, unsigned long start,
>
> void flush_tlb_mm(struct mm_struct *mm)
> {
> - __sbi_tlb_flush_range(mm_cpumask(mm), 0, -1);
> + __sbi_tlb_flush_range(mm, 0, -1);
> }
>
> void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
> {
> - __sbi_tlb_flush_range(mm_cpumask(vma->vm_mm), addr, PAGE_SIZE);
> + __sbi_tlb_flush_range(vma->vm_mm, addr, PAGE_SIZE);
> }
>
> void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
> unsigned long end)
> {
> - __sbi_tlb_flush_range(mm_cpumask(vma->vm_mm), start, end - start);
> + __sbi_tlb_flush_range(vma->vm_mm, start, end - start);
> }

LGTM. I took the first one as IMO they're really distnict issues, LMK
if you want to re-spin this one or if I should just take what's here.