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

[Alexandre Ghiti]

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@xxxxxxxxxx>
> > > ---
> > >    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@xxxxxxxxxxxx>

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
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@xxxxxxxxxxxxxxxxxxx
> http://lists.infradead.org/mailman/listinfo/linux-riscv