linux-kernel - Re: [External] [PATCH v6 05/13] riscv: Only send remote fences when some other CPU is online

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Date: Wed, 27 Mar 2024 14:16:00 +0800
From: yunhui cui <cuiyunhui@...edance.com>
To: Samuel Holland <samuel.holland@...ive.com>
Cc: Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Alexandre Ghiti <alexghiti@...osinc.com>, Jisheng Zhang <jszhang@...nel.org>
Subject: Re: [External] [PATCH v6 05/13] riscv: Only send remote fences when
 some other CPU is online

Hi Samuel,

On Wed, Mar 27, 2024 at 12:50 PM Samuel Holland
<samuel.holland@...ive.com> 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@...ive.com>
> ---
>
> (no changes since v4)
>
> 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 d76fc73e594b..f5be1fec8191 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)
> +               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 da821315d43e..0901aa47b58f 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -79,7 +79,9 @@ static void __ipi_flush_tlb_all(void *info)
>
>  void flush_tlb_all(void)
>  {
> -       if (riscv_use_sbi_for_rfence())
> +       if (num_online_cpus() < 2)
> +               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);
> --
> 2.43.1
>

>From a perceptual point of view, the modification here is not
necessary. There is such logic in on_each_cpu(). Can you share your
test data?


Thanks,
Yunhui