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

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Sun, 22 Sep 2024 16:39:53 +0530

Luming Yu <[email-masked]> writes:

> From: Yu Luming <[email-masked]>
>
> 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.

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.

-ritesh

>
> Signed-off-by: Luming Yu <[email-masked]>
> ---
>  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