Re: [PATCH 2/2] riscv: tlb: avoid tlb flushing if fullmm == 1

[Alexandre Ghiti <alex@ghiti.fr>]

On 02/01/2024 04:12, Jisheng Zhang wrote:
> On Sat, Dec 30, 2023 at 07:26:11PM +0100, Alexandre Ghiti wrote:
>> Hi Jisheng,
> Hi Alex,
>
>> On 28/12/2023 09:46, Jisheng Zhang wrote:
>>> The mmu_gather code sets fullmm=1 when tearing down the entire address
>>> space for an mm_struct on exit or execve. So if the underlying platform
>>> supports ASID, the tlb flushing can be avoided because the ASID
>>> allocator will never re-allocate a dirty ASID.
>>>
>>> Use the performance of Process creation in unixbench on T-HEAD TH1520
>>> platform is improved by about 4%.
>>>
>>> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
>>> ---
>>>    arch/riscv/include/asm/tlb.h | 9 +++++++++
>>>    1 file changed, 9 insertions(+)
>>>
>>> diff --git a/arch/riscv/include/asm/tlb.h b/arch/riscv/include/asm/tlb.h
>>> index 1eb5682b2af6..35f3c214332e 100644
>>> --- a/arch/riscv/include/asm/tlb.h
>>> +++ b/arch/riscv/include/asm/tlb.h
>>> @@ -12,10 +12,19 @@ static void tlb_flush(struct mmu_gather *tlb);
>>>    #define tlb_flush tlb_flush
>>>    #include <asm-generic/tlb.h>
>>> +#include <asm/mmu_context.h>
>>>    static inline void tlb_flush(struct mmu_gather *tlb)
>>>    {
>>>    #ifdef CONFIG_MMU
>>> +	/*
>>> +	 * If ASID is supported, the ASID allocator will either invalidate the
>>> +	 * ASID or mark it as used. So we can avoid TLB invalidation when
>>> +	 * pulling down a full mm.
>>> +	 */
>>
>> Given the number of bits are limited for the ASID, at some point we'll reuse
>> previously allocated ASID so the ASID allocator must make sure to invalidate
>> the entries when reusing an ASID: can you point where this is done?
> Per my understanding of the code, the path would be
> set_mm_asid()
>    __new_context()
>      __flush_context()  // set context_tlb_flush_pending
> if (need_flush_tlb)
>    local_flush_tlb_all()


Ok thanks, so feel free to add:

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks!

Alex


>
> Thanks
>   
>>> +	if (static_branch_likely(&use_asid_allocator) && tlb->fullmm)
>>> +		return;
>>> +
>>>    	if (tlb->fullmm || tlb->need_flush_all)
>>>    		flush_tlb_mm(tlb->mm);
>>>    	else
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