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Message-ID: <5693310f-8706-42e3-89e5-af512a24ff99@huawei.com>
Date: Wed, 26 Jun 2024 20:41:19 +0800
From: "liuyuntao (F)" <liuyuntao12@...wei.com>
To: Andrew Jones <ajones@...tanamicro.com>
CC: <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
<paul.walmsley@...ive.com>, <palmer@...belt.com>, <aou@...s.berkeley.edu>,
<liuyuntao12@...wei.com>
Subject: Re: [PATCH] RISC-V: Dynamically allocate cpumasks and further
increase range and default value of NR_CPUS
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@...wei.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
>>>
>>>
>>> _______________________________________________
>>> linux-riscv mailing list
>>> linux-riscv@...ts.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
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