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

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Date: Thu, 28 Mar 2024 10:21:37 +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 Thu, Mar 28, 2024 at 4:14 AM Samuel Holland
<samuel.holland@...ive.com> wrote:
>
> Hi Yunhui,
>
> On 2024-03-27 1:16 AM, yunhui cui wrote:
> > 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?
>
> The logic in on_each_cpu() doesn't apply when riscv_use_sbi_for_rfence() is
> true, so we would make unnecessary SBI calls, and cannot be oppimized out when
> CONFIG_SMP=n.

Is it possible to do this:
"sbi_remote_sfence_vma_asid(cpu_online_mask,...); " instead of adding:
"if (num_online_cpus() < 2)" ?

Thanks,
Yunhui