RISCV: the machanism of available_harts may cause other harts boot failure

[Heinrich Schuchardt]

On 9/5/22 17:30, Sean Anderson wrote:
> On 9/5/22 3:47 AM, Nikita Shubin wrote:
>> Hi Rick!
>>
>> On Mon, 5 Sep 2022 14:22:41 +0800
>> Rick Chen <rickchen36 at gmail.com> wrote:
>>
>>> Hi,
>>>
>>> When I free-run a SMP system, I once hit a failure case where some
>>> harts didn't boot to the kernel shell successfully.
>>> However it can't be duplicated anymore even if I try many times.
>>>
>>> But when I set a break during debugging with GDB, it can trigger the
>>> failure case each time.
>>
>> If hart fails to register itself to available_harts before
>> send_ipi_many is hit by the main hart:
>> https://elixir.bootlin.com/u-boot/v2022.10-rc3/source/arch/riscv/lib/smp.c#L50
>>
>> it won't exit the secondary_hart_loop:
>> https://elixir.bootlin.com/u-boot/v2022.10-rc3/source/arch/riscv/cpu/start.S#L433
>> As no ipi will be sent to it.

Can we call send_ipi_many() again when booting?
Do we need to call it before booting?

Best regards

Heinrich

>>
>> This might be exactly your case.
>
> When working on the IPI mechanism, I considered this possibility. However,
> there's really no way to know how long to wait. On normal systems, the boot
> hart is going to do a lot of work before calling send_ipi_many, and the
> other harts just have to make it through ~100 instructions. So I figured we
> would never run into this issue.
>
> We might not even need the mask... the only direct reason we might is for
> OpenSBI, as spl_invoke_opensbi is the only function which uses the wait
> parameter.
>
>>> I think the mechanism of available_harts does not provide a method
>>> that guarantees the success of the SMP system.
>>> Maybe we shall think of a better way for the SMP booting or just
>>> remove it ?
>>
>> I haven't experienced any unexplained problem with hart_lottery or
>> available_harts_lock unless:
>>
>> 1) harts are started non-simultaneously
>> 2) SPL/U-Boot is in some kind of TCM, OCRAM, etc... which is not cleared
>> on reset which leaves available_harts dirty
>
> XIP, of course, has this problem every time and just doesn't use the mask.
> I remember thinking a lot about how to deal with this, but I never ended
> up sending a patch because I didn't have a XIP system.
>
> --Sean
>
>> 3) something is wrong with atomics
>>
>> Also there might be something wrong with IPI send/recieve.
>>
>>>
>>> Thread 8 hit Breakpoint 1, harts_early_init ()
>>>
>>> (gdb) c
>>> Continuing.
>>> [Switching to Thread 7]
>>>
>>> Thread 7 hit Breakpoint 1, harts_early_init ()
>>>
>>> (gdb)
>>> Continuing.
>>> [Switching to Thread 6]
>>>
>>> Thread 6 hit Breakpoint 1, harts_early_init ()
>>>
>>> (gdb)
>>> Continuing.
>>> [Switching to Thread 5]
>>>
>>> Thread 5 hit Breakpoint 1, harts_early_init ()
>>>
>>> (gdb)
>>> Continuing.
>>> [Switching to Thread 4]
>>>
>>> Thread 4 hit Breakpoint 1, harts_early_init ()
>>>
>>> (gdb)
>>> Continuing.
>>> [Switching to Thread 3]
>>>
>>> Thread 3 hit Breakpoint 1, harts_early_init ()
>>> (gdb)
>>> Continuing.
>>> [Switching to Thread 2]
>>>
>>> Thread 2 hit Breakpoint 1, harts_early_init ()
>>> (gdb)
>>> Continuing.
>>> [Switching to Thread 1]
>>>
>>> Thread 1 hit Breakpoint 1, harts_early_init ()
>>> (gdb)
>>> Continuing.
>>> [Switching to Thread 5]
>>>
>>>
>>> Thread 5 hit Breakpoint 3, 0x0000000001200000 in ?? ()
>>> (gdb) info threads
>>>    Id   Target Id         Frame
>>>    1    Thread 1 (hart 1) secondary_hart_loop () at
>>> arch/riscv/cpu/start.S:436 2    Thread 2 (hart 2) secondary_hart_loop
>>> () at arch/riscv/cpu/start.S:436 3    Thread 3 (hart 3)
>>> secondary_hart_loop () at arch/riscv/cpu/start.S:436 4    Thread 4
>>> (hart 4) secondary_hart_loop () at arch/riscv/cpu/start.S:436
>>> * 5    Thread 5 (hart 5) 0x0000000001200000 in ?? ()
>>>    6    Thread 6 (hart 6) 0x000000000000b650 in ?? ()
>>>    7    Thread 7 (hart 7) 0x000000000000b650 in ?? ()
>>>    8    Thread 8 (hart 8) 0x0000000000005fa0 in ?? ()
>>> (gdb) c
>>> Continuing.
>>
>> Do they all "offline" harts remain in SPL/U-Boot secondary_hart_loop ?
>>
>>>
>>>
>>>
>>> [    0.175619] smp: Bringing up secondary CPUs ...
>>> [    1.230474] CPU1: failed to come online
>>> [    2.282349] CPU2: failed to come online
>>> [    3.334394] CPU3: failed to come online
>>> [    4.386783] CPU4: failed to come online
>>> [    4.427829] smp: Brought up 1 node, 4 CPUs
>>>
>>>
>>> /root # cat /proc/cpuinfo
>>> processor       : 0
>>> hart            : 4
>>> isa     : rv64i2p0m2p0a2p0c2p0xv5-1p1
>>> mmu             : sv39
>>>
>>> processor       : 5
>>> hart            : 5
>>> isa     : rv64i2p0m2p0a2p0c2p0xv5-1p1
>>> mmu             : sv39
>>>
>>> processor       : 6
>>> hart            : 6
>>> isa     : rv64i2p0m2p0a2p0c2p0xv5-1p1
>>> mmu             : sv39
>>>
>>> processor       : 7
>>> hart            : 7
>>> isa     : rv64i2p0m2p0a2p0c2p0xv5-1p1
>>> mmu             : sv39
>>>
>>> /root #
>>>
>>> Thanks,
>>> Rick
>>
>