linux-kernel - Re: [PATCH v4 04/12] riscv: Only send remote fences when some other CPU is online

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Date: Thu, 4 Jan 2024 13:33:57 +0100
From: Alexandre Ghiti <alexghiti@...osinc.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
Subject: Re: [PATCH v4 04/12] riscv: Only send remote fences when some other
 CPU is online

On Tue, Jan 2, 2024 at 11:01 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>
> ---
>
> 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)
> +               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)
> +               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
>

on_each_cpu() already deals correctly with a single online cpu, the
only thing to optimize here is the SBI rfence. So I'd move this new
test in sbi_remote_sfence_vma_asid() and sbi_remote_fence_i() to avoid
the superfluous M-mode entry when only one cpu is online by checking
the cpumask. And since sbi_remote_fence_i() is used in another
function (flush_icache_mm()), we could also take advantage of this
optimization when only the local cpu must be flushed.