linux-kernel - Re: [PATCH 2/4] irq_work: Provide a irq work that can be processed on any cpu

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Message-ID: <20131107223249.GB28130@localhost.localdomain>
Date:	Thu, 7 Nov 2013 23:32:51 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/4] irq_work: Provide a irq work that can be processed
 on any cpu

On Thu, Nov 07, 2013 at 11:19:04PM +0100, Jan Kara wrote:
> On Thu 07-11-13 23:13:39, Frederic Weisbecker wrote:
> > But then, who's going to process that work if every CPUs is idle?
>   Have a look into irq_work_queue(). There is:
>         /*
>          * If the work is not "lazy" or the tick is stopped, raise the irq
>          * work interrupt (if supported by the arch), otherwise, just wait
>          * for the next tick. We do this even for unbound work to make sure
>          * *some* CPU will be doing the work.
>          */
>         if (!(work->flags & IRQ_WORK_LAZY) || tick_nohz_tick_stopped()) {
>                 if (!this_cpu_cmpxchg(irq_work_raised, 0, 1))
>                         arch_irq_work_raise();
>         }
> 
>   So we raise an interrupt if there would be no timer ticking (which is
> what I suppose you mean by "CPU is idle"). That is nothing changed by my
> patches...

That said I agree that it would be nice to have smp_call_function_many() support
non waiting calls, something based on llist, that would be less deadlock prone
to begin with.
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