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

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Date:	Thu, 7 Nov 2013 23:54:10 +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 11:50:34PM +0100, Jan Kara wrote:
> On Thu 07-11-13 23:23:14, Frederic Weisbecker wrote:
> > 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...
> > 
> > Ok but we raise that interrupt locally, not to the other CPUs.
>   True, but that doesn't really matter in this case. Any CPU (including the
> local one) can handle the unbound work. So from the definition of the
> unbound work things are OK.

I don't see how that can be ok. You want to offline a work because the local CPU
can't handle it, right? If the local CPU can handle it you can just use local
irq works.

> 
> Regarding my use for printk - if all (other) CPUs are idle then we can
> easily afford making the current cpu busy printing, that's not a problem.
> There's nothing else to do than to print what's remaining in the printk
> buffer...

So if the current CPU can handle it, what is the problem?
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