Re: [PATCH] RISC-V: Dynamically allocate cpumasks and further increase range and default value of NR_CPUS

["liuyuntao (F)" <liuyuntao12@huawei.com>]

Gentle ping

On 2024/6/26 20:41, liuyuntao (F) wrote:
> 
> 
> On 2024/6/25 19:44, liuyuntao (F) wrote:
>>
>>
>> On 2024/6/25 19:11, Andrew Jones wrote:
>>> On Fri, Jun 14, 2024 at 07:53:06AM GMT, Yuntao Liu wrote:
>>>> Currently default NR_CPUS is 64 for riscv64, since the latest QEMU virt
>>>> machine supports up to 512 CPUS, so set default NR_CPUS 512 for 
>>>> riscv64.
>>>>
>>>> Under the promotion of RISC-V International and related chip
>>>> manufacturers, RISC-V has also begun to enter the server market, which
>>>> demands higher performance. Other major architectures (such as ARM64,
>>>> x86_64, MIPS, etc) already have a higher range, so further increase
>>>> this range up to 4096 for riscv64.
>>>>
>>>> Due to the fact that increasing NR_CPUS enlarges the size of cpumasks,
>>>> there is a concern that this could significantly impact stack usage,
>>>> especially for code that allocates cpumasks on the stack. To address
>>>> this, we have the option to enable CPUMASK_OFFSTACK, which prevents
>>>> cpumasks from being allocated on the stack. we choose to enable this
>>>> feature only when NR_CPUS is greater than 512, why 512, since then
>>>> the kernel size with offstack is smaller.
>>>
>>> This isn't the reason why Arm decided to start at 512, afaict. The 
>>> reason
>>> for Arm was because hackbench did better with onstack for 256. What are
>>> the hackbench results for riscv?
>>
>> Okay, I will add the test results of hacktest soon.
> 
> Benchmark results using hackbench average over 5 runs of
> ./hackbench -s 512 -l 20 -g 10 -f 50 -P
> on Qemu.
> 
> NR_CPUS     64      128     256     512     1024    2048
> onstack/s   6.9992  6.6112  6.7834  6.6578  6.6646  6.8692
> offstack/s  6.5616  6.95    6.5698  6.91    6.663   6.8202
> difference  -6.25%  +5.12%  -3.15%  +3.79%  -0.02%  -0.71%
> 
> When there are more cores, the fluctuation is minimal, leading to the 
> speculation that the performance gap would be smaller with a higher 
> number of NR_CPUS.
> Since I don't have a RISCV single-board computer, these are the results 
> I obtained from testing in QEMU, which may differ from the actual 
> situation. Perhaps someone could help with the testing.
> 
> Thanks,
> Yuntao
> 
>>
>>>
>>>>
>>>> vmlinux size comparison(difference to vmlinux_onstack_NR_CPUS 
>>>> baseline):
>>>>
>>>> NR_CPUS     256         512         1024        2048        4096
>>>> onstack     19814536    19840760    19880584    19969672    20141704
>>>> offstack    19819144    19840936    19880480    19968544    20135456
>>>> difference  +0.023%     +0.001%     -0.001%     -0.001      -0.031%
>>>> is_smaller  n           n           y           y           y
>>>
>>> Since the savings are almost nothing we must not have too many global
>>> cpumasks. But I'm in favor of ensuring stack depths stay under control,
>>> so turning on CPUMASK_OFFSTACK sounds good to me in general.
>>>
>>>>
>>>> Signed-off-by: Yuntao Liu <liuyuntao12@huawei.com>
>>>> ---
>>>>   arch/riscv/Kconfig | 5 +++--
>>>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>>> index 0525ee2d63c7..5960713b3bf9 100644
>>>> --- a/arch/riscv/Kconfig
>>>> +++ b/arch/riscv/Kconfig
>>>> @@ -77,6 +77,7 @@ config RISCV
>>>>       select CLINT_TIMER if RISCV_M_MODE
>>>>       select CLONE_BACKWARDS
>>>>       select COMMON_CLK
>>>> +    select CPUMASK_OFFSTACK if NR_CPUS > 512
>>>>       select CPU_PM if CPU_IDLE || HIBERNATION || SUSPEND
>>>>       select EDAC_SUPPORT
>>>>       select FRAME_POINTER if PERF_EVENTS || (FUNCTION_TRACER && 
>>>> !DYNAMIC_FTRACE)
>>>> @@ -428,11 +429,11 @@ config SCHED_MC
>>>>   config NR_CPUS
>>>>       int "Maximum number of CPUs (2-512)"
>>>>       depends on SMP
>>>> -    range 2 512 if !RISCV_SBI_V01
>>>> +    range 2 4096 if !RISCV_SBI_V01
>>>>       range 2 32 if RISCV_SBI_V01 && 32BIT
>>>>       range 2 64 if RISCV_SBI_V01 && 64BIT
>>>>       default "32" if 32BIT
>>>> -    default "64" if 64BIT
>>>> +    default "512" if 64BIT
>>>
>>> This is somewhat reasonable, even if nothing is going to use this for
>>> quite a while, since it'll help avoid bugs popping up when NR_CPUS gets
>>> bumped later, but it feels excessive right now for riscv, so I'm a bit
>>> on the fence about it. Maybe if hackbench doesn't show any issues we
>>> could turn CPUMASK_OFFSTACK on for a smaller NR_CPUS and also select
>>> a smaller default?
>>>
> 
> It seems that when NR_CPUS is larger, hackbench performs better, and 
> which NR_CPUS do you have a preference for?
> 
>>> Thanks,
>>> drew
>>>
>>>>   config HOTPLUG_CPU
>>>>       bool "Support for hot-pluggable CPUs"
>>>> -- 
>>>> 2.34.1
>>>>
>>>>
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