Re: [RFC PATCH] powerpc/tlb: enable arch want batched unmap tlb flush

[Luming Yu]

On Sun, Sep 22, 2024 at 04:39:53PM +0530, Ritesh Harjani wrote:
> Luming Yu <luming.yu@xxxxxxxxxxxx> writes:
> 
> > From: Yu Luming <luming.yu@xxxxxxxxx>
> >
> > ppc always do its own tracking for batch tlb. By trivially enabling
> > the ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH in ppc, ppc arch can re-use
> > common code in rmap and reduce overhead and do optimization it could not
> > have without a tlb flushing context at low architecture level.
> 
> I looked at this patch and other than the compile failure, this patch
> still won't optimize anything. The idea of this config is that we want
> to batch all the tlb flush operation at the end. By returning false from
> should_defer_flush() (in this patch), we are saying we cannot defer
> the flush and hence we do tlb flush in the same context of unmap.
not exactly, as false return implies, we currently do nothing but relying on
book3S_64's tlb batch implementation which contains a bit of defer optimization
that we need to use a real benchmark to do some performance characterization.

And I need to get my test bed ready for patch testing first. So I have to
defer the real optimization in this area.
> 
> Anyway, I took a quick look at ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> and I have a quick PoC for the same. I will soon post it.
thanks for picking up the barton for the future collaboration on the
potential common performance benefits among us for powerpc arch.
> 
> -ritesh
> 
> >
> > Signed-off-by: Luming Yu <luming.yu@xxxxxxxxxxxx>
> > ---
> >  arch/powerpc/Kconfig                |  1 +
> >  arch/powerpc/include/asm/tlbbatch.h | 30 +++++++++++++++++++++++++++++
> >  2 files changed, 31 insertions(+)
> >  create mode 100644 arch/powerpc/include/asm/tlbbatch.h
> >
> > diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> > index e94e7e4bfd40..e6db84dd014a 100644
> > --- a/arch/powerpc/Kconfig
> > +++ b/arch/powerpc/Kconfig
> > @@ -175,6 +175,7 @@ config PPC
> >  	select ARCH_WANT_IPC_PARSE_VERSION
> >  	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
> >  	select ARCH_WANT_LD_ORPHAN_WARN
> > +	select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> >  	select ARCH_WANT_OPTIMIZE_DAX_VMEMMAP	if PPC_RADIX_MMU
> >  	select ARCH_WANTS_MODULES_DATA_IN_VMALLOC	if PPC_BOOK3S_32 || PPC_8xx
> >  	select ARCH_WEAK_RELEASE_ACQUIRE
> > diff --git a/arch/powerpc/include/asm/tlbbatch.h b/arch/powerpc/include/asm/tlbbatch.h
> > new file mode 100644
> > index 000000000000..484628460057
> > --- /dev/null
> > +++ b/arch/powerpc/include/asm/tlbbatch.h
> > @@ -0,0 +1,30 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ARCH_PPC_TLBBATCH_H
> > +#define _ARCH_PPC_TLBBATCH_H
> > +
> > +struct arch_tlbflush_unmap_batch {
> > +	/*
> > +         *
> > +	 */
> > +};
> > +
> > +static inline void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
> > +{
> > +}
> > +
> > +static inline void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
> > +						struct mm_struct *mm,
> > +						unsigned long uarddr)
> > +{
> > +}
> > +
> > +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> > +{
> > +	/*ppc always do tlb flush in batch*/
> > +	return false;
> > +}
> > +
> > +static inline void arch_flush_tlb_batched_pending(struct mm_struct *mm)
> > +{
> > +}
> > +#endif /* _ARCH_PPC_TLBBATCH_H */
> > -- 
> > 2.42.0.windows.2
>