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 15:14:41 -0500
From: Samuel Holland <samuel.holland@...ive.com>
To: yunhui cui <cuiyunhui@...edance.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 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. The cover letter includes benchmarks for a representative
single-core system (D1). There was no measurable performance impact from this
portion of the series on multi-core systems. If there are specific benchmarks
you think I should run, please let me know.

Regards,
Samuel