Re: [PATCH v2] MIPS: smp: report dying CPU to RCU in stop_this_cpu()

[Jonas Jelonek <jelonek.jonas@gmail.com>]

Hi Huacai,

On 15.06.26 09:09, Huacai Chen wrote:
> On Mon, Jun 15, 2026 at 3:00 PM Jonas Jelonek <jelonek.jonas@gmail.com> wrote:
>> Hi Huacai,
>>
>> sorry for the reply delay.
>>
>> On 10.06.26 08:05, Huacai Chen wrote:
>>> [...]
>>> In theory LoongArch has the same problem, but I cannot reproduce,
>>> should I enable PREEMPT_RT? Or there are some special configurations?
>> Sadly I cannot help with that. For MIPS, this seems to be the default
>> behavior.
> This patch fixes 91840be8f710, and 91840be8f710 adds synchronize_rcu()
> in irq_work_sync(). Your problem is caused by this synchronize_rcu(),
> right?

Yes it is.

> However, synchronize_rcu() only gets called in the
> IS_ENABLED(CONFIG_PREEMPT_RT) case, so I think your configuration
> needs PREEMPT_RT, right?
>
> You said this is the default behavior, but PREEMPT_RT is not enabled by default.

The condition where this is added has two parts, see [1]. While PREEMPT_RT
isn't active for MIPS, arch_irq_work_has_interrupt gives false for MIPS (since
there is no implementation and it falls back to the generic one). This then
also calls synchronize_rcu.

> Huacai
>

Best,
Jonas

[1] https://elixir.bootlin.com/linux/v7.1-rc7/source/kernel/irq_work.c#L291-L302