linux-kernel - Re: [RFC 4/6] softirq: Run per-group per-cpu ksoftirqd thread

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Date:   Thu, 18 Jan 2018 18:00:03 +0100
From:   Mike Galbraith <efault@....de>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radu Rendec <rrendec@...sta.com>,
        Rik van Riel <riel@...hat.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [RFC 4/6] softirq: Run per-group per-cpu ksoftirqd thread

On Thu, 2018-01-18 at 16:12 +0000, Dmitry Safonov wrote:
> 
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index 2ea09896bd6e..17e1a04445fa 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -508,11 +508,21 @@ extern void __raise_softirq_irqoff(unsigned int nr);
>  extern void raise_softirq_irqoff(unsigned int nr);
>  extern void raise_softirq(unsigned int nr);
>  
> -DECLARE_PER_CPU(struct task_struct *, ksoftirqd);
> +extern struct task_struct *__percpu **ksoftirqd;
> +extern unsigned nr_softirq_groups;
>  
> -static inline struct task_struct *this_cpu_ksoftirqd(void)
> +extern bool servicing_softirq(unsigned nr);
> +static inline bool current_is_ksoftirqd(void)
>  {
> -	return this_cpu_read(ksoftirqd);
> +	unsigned i;
> +
> +	if (!ksoftirqd)
> +		return false;
> +
> +	for (i = 0; i < nr_softirq_groups; i++)
> +		if (*this_cpu_ptr(ksoftirqd[i]) ==
> current)
> +			return true;
> +	return false;
>  }

I haven't read all this, but in a quick drive-by this poked me in the
eye.  For RT tree fully threaded softirqs, I stole a ->flags bit to
identify threads ala PF_KTHREAD (PF_KSOFTIRQD).  In previous versions,
I added a bit field to do the same, either is quicker than rummaging.

	-Mike