linux-kernel - Re: [PATCH v3 5/6] irq_work: Provide irq_work_queue

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Message-ID: <20201028134046.GE229044@lothringen>
Date:   Wed, 28 Oct 2020 14:40:46 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, will@...nel.org,
        paulmck@...nel.org, hch@....de, axboe@...nel.dk,
        chris@...is-wilson.co.uk, davem@...emloft.net, kuba@...nel.org,
        fweisbec@...il.com, oleg@...hat.com, vincent.guittot@...aro.org
Subject: Re: [PATCH v3 5/6] irq_work: Provide irq_work_queue_remote()

On Wed, Oct 28, 2020 at 12:07:12PM +0100, Peter Zijlstra wrote:
> While the traditional irq_work relies on the ability to self-IPI, it
> makes sense to provide an unconditional irq_work_queue_remote()
> interface.

We may need a reason as well here.

> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1308,13 +1308,14 @@ static int rcu_implicit_dynticks_qs(stru
>  			resched_cpu(rdp->cpu);
>  			WRITE_ONCE(rdp->last_fqs_resched, jiffies);
>  		}
> -		if (IS_ENABLED(CONFIG_IRQ_WORK) &&
> -		    !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
> +#ifdef CONFIG_IRQ_WORK
> +		if (!rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&

If it's unconditional on SMP, I expect it to be unconditional on rcutree.

Also this chunk seems unrelated to this patch.

>  		    (rnp->ffmask & rdp->grpmask)) {
>  			rdp->rcu_iw_pending = true;
>  			rdp->rcu_iw_gp_seq = rnp->gp_seq;
>  			irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
>  		}
> +#endif
>  	}
>  
>  	return 0;
> 
>