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

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

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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