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Date: Wed, 3 Jan 2024 22:57:58 +0800
From: Jisheng Zhang <jszhang@kernel.org>
To: Samuel Holland <samuel.holland@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>, linux-riscv@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Alexandre Ghiti <alexghiti@rivosinc.com>
Subject: Re: [PATCH v4 04/12] riscv: Only send remote fences when some other
 CPU is online
Message-ID: <ZZV19owFbjOzgG89@xhacker>
References: <20240102220134.3229156-1-samuel.holland@sifive.com>
 <20240102220134.3229156-5-samuel.holland@sifive.com>
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In-Reply-To: <20240102220134.3229156-5-samuel.holland@sifive.com>

On Tue, Jan 02, 2024 at 02:00:41PM -0800, Samuel Holland wrote:
> If no other CPU is online, a local cache or TLB flush is sufficient.
> These checks can be constant-folded when SMP is disabled.
> 
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
> ---
> 
> Changes in v4:
>  - New patch for v4
> 
>  arch/riscv/mm/cacheflush.c | 4 +++-
>  arch/riscv/mm/tlbflush.c   | 4 +++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> index 47c485bc7df0..f7933ae88a55 100644
> --- a/arch/riscv/mm/cacheflush.c
> +++ b/arch/riscv/mm/cacheflush.c
> @@ -21,7 +21,9 @@ void flush_icache_all(void)
>  {
>  	local_flush_icache_all();
>  
> -	if (riscv_use_sbi_for_rfence())
> +	if (num_online_cpus() < 2)

with patch5, I think it's better to short cut for !SMP, I.E
if (!IS_ENABLED(CONFIG_SMP) || num_online_cpus()) < 2)

so that the UP case can avoid a atomic read and check

> +		return;
> +	else if (riscv_use_sbi_for_rfence())
>  		sbi_remote_fence_i(NULL);
>  	else
>  		on_each_cpu(ipi_remote_fence_i, NULL, 1);
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index 2f18fe6fc4f3..37b3c93e3c30 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -73,7 +73,9 @@ static void __ipi_flush_tlb_all(void *info)
>  
>  void flush_tlb_all(void)
>  {
> -	if (riscv_use_sbi_for_rfence())
> +	if (num_online_cpus() < 2)

ditto

> +		local_flush_tlb_all();
> +	else if (riscv_use_sbi_for_rfence())
>  		sbi_remote_sfence_vma_asid(NULL, 0, FLUSH_TLB_MAX_SIZE, FLUSH_TLB_NO_ASID);
>  	else
>  		on_each_cpu(__ipi_flush_tlb_all, NULL, 1);
> -- 
> 2.42.0
> 
> 
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