linux-kernel - Re: [PATCH v3 05/17] riscv: misaligned: use on_each_cpu() for scalar misaligned access probing

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Message-ID: <1942580f-cd67-4ddd-b489-0532f95c1ef2@rivosinc.com>
Date: Fri, 14 Mar 2025 12:44:05 +0100
From: Clément Léger <cleger@...osinc.com>
To: Andrew Jones <ajones@...tanamicro.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Anup Patel <anup@...infault.org>,
 Atish Patra <atishp@...shpatra.org>, Shuah Khan <shuah@...nel.org>,
 Jonathan Corbet <corbet@....net>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
 linux-kselftest@...r.kernel.org, Samuel Holland <samuel.holland@...ive.com>
Subject: Re: [PATCH v3 05/17] riscv: misaligned: use on_each_cpu() for scalar
 misaligned access probing



On 13/03/2025 13:57, Andrew Jones wrote:
> On Mon, Mar 10, 2025 at 04:12:12PM +0100, Clément Léger wrote:
>> schedule_on_each_cpu() was used without any good reason while documented
>> as very slow. This call was in the boot path, so better use
>> on_each_cpu() for scalar misaligned checking. Vector misaligned check
>> still needs to use schedule_on_each_cpu() since it requires irqs to be
>> enabled but that's less of a problem since this code is ran in a kthread.
>> Add a comment to explicit that.
>>
>> Signed-off-by: Clément Léger <cleger@...osinc.com>
>> ---
>>  arch/riscv/kernel/traps_misaligned.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
>> index 90ac74191357..ffac424faa88 100644
>> --- a/arch/riscv/kernel/traps_misaligned.c
>> +++ b/arch/riscv/kernel/traps_misaligned.c
>> @@ -616,6 +616,11 @@ bool check_vector_unaligned_access_emulated_all_cpus(void)
>>  		return false;
>>  	}
>>  
>> +	/*
>> +	 * While being documented as very slow, schedule_on_each_cpu() is used
>> +	 * since kernel_vector_begin() expects irqs to be enabled or it will panic().
> 
> which expects

Hum that would yield the following:

"schedule_on_each_cpu() is used since kernel_vector_begin() that is
called inside the vector code 'which' expects irqs to be enabled or it
will panic()." which seems wrong as well.

I guess something like this would be better:

"While being documented as very slow, schedule_on_each_cpu() is used
since kernel_vector_begin() expects irqs to be enabled or it will panic()"

Thanks,

Clément

> 
>> +	 */
>>  	schedule_on_each_cpu(check_vector_unaligned_access_emulated);
>>  
>>  	for_each_online_cpu(cpu)
>> @@ -636,7 +641,7 @@ bool check_vector_unaligned_access_emulated_all_cpus(void)
>>  
>>  static bool unaligned_ctl __read_mostly;
>>  
>> -static void check_unaligned_access_emulated(struct work_struct *work __always_unused)
>> +static void check_unaligned_access_emulated(void *arg __always_unused)
>>  {
>>  	int cpu = smp_processor_id();
>>  	long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
>> @@ -677,7 +682,7 @@ bool check_unaligned_access_emulated_all_cpus(void)
>>  	 * accesses emulated since tasks requesting such control can run on any
>>  	 * CPU.
>>  	 */
>> -	schedule_on_each_cpu(check_unaligned_access_emulated);
>> +	on_each_cpu(check_unaligned_access_emulated, NULL, 1);
>>  
>>  	for_each_online_cpu(cpu)
>>  		if (per_cpu(misaligned_access_speed, cpu)
>> -- 
>> 2.47.2
>>
> 
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>