Re: [PATCH v5 1/3] genirq: Use hlist for managing resend handlers

[Liao, Chang]

Hi, Thomas

在 2023/5/30 5:51, Thomas Gleixner 写道:
> On Mon, May 29 2023 at 15:57, Chang Liao wrote:
>> 在 2023/5/19 21:49, Shanker Donthineni 写道:
>>> diff --git a/include/linux/irqdesc.h b/include/linux/irqdesc.h
>>> index 844a8e30e6de..d9451d456a73 100644
>>> --- a/include/linux/irqdesc.h
>>> +++ b/include/linux/irqdesc.h
>>> @@ -102,6 +102,9 @@ struct irq_desc {
>>>  	int			parent_irq;
>>>  	struct module		*owner;
>>>  	const char		*name;
>>> +#ifdef CONFIG_HARDIRQS_SW_RESEND
>>> +	struct hlist_node	resend_node;
>>> +#endif
>>>  } ____cacheline_internodealigned_in_smp;
>>
>> Although there is no documented rule that limits the change of the KABI
>> struct irq_desc, it is still better to keep the irq_desc definition
>> stable.
> 
> Please read and understand:
> 
>        Documentation/process/stable-api-nonsense.rst
> 
> If you want KABI, then that's  _YOUR_ problem, period.

Thanks for the guide.

> 
>>> -/* Bitmap to handle software resend of interrupts: */
>>> -static DECLARE_BITMAP(irqs_resend, IRQ_BITMAP_BITS);
>>> +/* hlist_head to handle software resend of interrupts: */
>>> +static HLIST_HEAD(irq_resend_list);
>>> +static DEFINE_RAW_SPINLOCK(irq_resend_lock);
>>
>> What is the benefit of using hlist here? If you want to enjoy the
>> low latency of querying elements by key, you must define a hlist table
>> with a reasonable number of buckets. Otherwise, I don't think the time
>> complexity of hlist is better than a regular double-linked list,
>> right?
> 
> What's complex about hlist in this case? Please explain.

Honestly, it is not about the complexity. Perhaps I do not understand the
usage of hlist very deeply. I have searched some codes in the kernel and
found that hlist is always used to speed up arbitrary querying, such as
searching a registered kprobe by address. Back to this patch, these resend
IRQs are organized in a sequence list actually, and traveled one by one to
handle. Further, by comparing the difference between hlist_empty, hlist_add_head,
hlist_del_init, and their counterparts in list, it looks like a regular linked
list is also good enough.

Thanks.

> 
> Thanks,
> 
>         tglx

-- 
BR
Liao, Chang