Re:Re: [PATCH,RESEND] softirq: Introduce SOFTIRQ_FORCED_THREADING

[王擎]

>Wang,
>
>On Mon, Aug 23 2021 at 11:33, Wang Qing wrote:
>
>> At present, whether the softirq is executed when the interrupt exits 
>> is controlled by IRQ_FORCED_THREADING. This is unreasonable. It should 
>> be split and allowed to take effect separately.
>
>There is nothing unreasonable about it. When interrupt force threading
>is in effect then it obviously requires that soft interrupt processing
>goes into threaded mode as well. But the threaded execution still takes
>place when the force threaded interrupt handler completes. Only softirqs
>which are raised from hard interrupt context (e.g. timer interrupt) are
>forced off to ksoftirqd.
>
>What you are proposing here is completly different as you enforce
>softirq execution in context of ksoftirqd only.

Thank you for reply and explanation, I just provide a choice to balance
the execution of softirq according to their own business scenarios.

>
>> At the same time, we should increase the priority of ksoftirqd when
>> forbidden to execute in interrupt exits. I refer to the implementation 
>> of PREEMPT_RT and think it is reasonable.
>
>What are you referring to? PREEMPT_RT does not modify the priority of
>ksoftirqd. If system designers want to do that, then they can do so from
>user space. 

I refer to the kernel-3.14 RT Patches. I used it at that time and achieved 
very good results.
I remember where I saw that softirqd was split into the original process 
and the RT process. This can partially solve my problem.

>
>And doing so can be problematic depending on the workload as this
>effectively breaks the softirq overload mitigation mechanism which
>depends on deferring to ksoftirqd so that e.g. the consumers of received
>network packets can be scheduled and the system can make progress.
>
>Just because it does not break on your system with your particular
>workload and configuration does not make it suitable for general
>consumption.
>
>> +#ifdef CONFIG_SOFTIRQ_FORCED_THREADING
>> +static inline void invoke_softirq(void)
>> +{
>> +	wakeup_softirqd();
>
>Depending on the configuration and timing this breaks any early boot
>mechanism which depends on softirqs being handled before ksoftirqd is
>available. This was clearly never tested with full RCU debugging
>enabled. 
>
>Aside of that the changelog lacks any form of technical analysis and
>justification for this. Just claiming that things are [un]reasonable and
>making uninformed statements about PREEMPT_RT does not qualify. Quite
>the contrary it's definitely unreasonable.
>
>Thanks,
>
>        tglx
>
>

Thank you for your patient guidance, if necessary, I will add it in the next version.

Thanks,
Qing