Re: [PATCH] riscv: mm: Limit per-page TLB flushes for large PTE batches

From: Jisheng Zhang <jszhang@kernel.org>
To: hu.shengming@zte.com.cn
Cc: pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu,
	alex@ghiti.fr, akpm@linux-foundation.org,
	baolin.wang@linux.alibaba.com, wangruikang@iscas.ac.cn,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	zhang.run@zte.com.cn, luo.haiyang@zte.com.cn
Subject: Re: [PATCH] riscv: mm: Limit per-page TLB flushes for large PTE batches
Date: Thu, 12 Nov 2026 20:00:06 +0800	[thread overview]
Message-ID: <avWqRtXhDwSDqlgs@xhacker> (raw)
In-Reply-To: <20261112110525378dgDlKhA-G74_y3b6ZeZkz@zte.com.cn>

On Thu, Nov 12, 2026 at 11:05:25AM +0800, hu.shengming@zte.com.cn wrote:
> From: Shengming Hu <hu.shengming@zte.com.cn>
> 
> On systems without Svvptc, update_mmu_cache_range() performs one local
> TLB invalidation per PTE. With 4 KiB pages, installing a PTE-mapped
> 1 MiB anonymous folio requires 256 SFENCE.VMA instructions, or 256
> SINVAL.VMA instructions bracketed by SFENCE.W.INVAL and
> SFENCE.INVAL.IR.
> 
> Apply the existing local TLB range-flush policy to PTE batch updates.
> When nr exceeds tlb_flush_all_threshold, use local_flush_tlb_all_asid()
> to replace per-page invalidations with a single ASID-wide SFENCE.VMA.
> This orders the PTE writes before subsequent implicit page-table
> accesses and invalidates translations for the entire batch. As with
> range flushes, this may also evict unrelated translations for the same
> ASID.
> 
> Benchmark results from will-it-scale's page_fault1_processes under
> QEMU 9.1.93 (qemu-9.2.0-0.3.rc3.fc40), with svvptc=false and
> CONFIG_TRANSPARENT_HUGEPAGE=y (higher is better):

Can you test on real hw rather than on qemu to get the performance numbers?

> 
> Folio size      Before       After     Change
> =============================================
> 4 KiB           116537      113503      -2.6%
> 512 KiB        1431962     1977519     +38.1%
> 1 MiB          1528457     2208209     +44.5%

Since one page is the most case, if we really regressed about 2.6%, then I
think we need to evaluate whether the gain deserves the pay.

> 
> Signed-off-by: Shengming Hu <hu.shengming@zte.com.cn>
> ---
>  arch/riscv/include/asm/pgtable.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 2ebd5fa5adc5..ebe42fcf35ae 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -571,6 +571,11 @@ static inline void update_mmu_cache_range(struct vm_fault *vmf,
>  	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SVVPTC))
>  		return;
> 
> +	if (nr > tlb_flush_all_threshold) {

unlikely(...)

> +		local_flush_tlb_all_asid(asid);
> +		return;
> +	}
> +
>  	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SVINVAL)) {
>  		local_sfence_w_inval();
>  		while (nr--)
> -- 
> 2.25.1
> 
> _______________________________________________
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> linux-riscv@lists.infradead.org
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