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

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Date:   Mon, 29 May 2023 23:51:02 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Liao, Chang" <liaochang1@...wei.com>,
        Shanker Donthineni <sdonthineni@...dia.com>,
        Marc Zyngier <maz@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Michael Walle <michael@...le.cc>, linux-kernel@...r.kernel.org,
        Vikram Sethi <vsethi@...dia.com>,
        Jason Sequeira <jsequeira@...dia.com>
Subject: Re: [PATCH v5 1/3] genirq: Use hlist for managing resend handlers

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.

>> -/* 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.

Thanks,

        tglx