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

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Date:	Fri, 8 Nov 2013 11:18:01 +0100
From:	Jan Kara <jack@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Kara <jack@...e.cz>, Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH 2/4] irq_work: Provide a irq work that can be processed
 on any cpu

On Thu 07-11-13 18:31:42, Steven Rostedt wrote:
> On Fri, 8 Nov 2013 00:01:11 +0100
> Jan Kara <jack@...e.cz> wrote:
> 
> > On Thu 07-11-13 23:54:10, Frederic Weisbecker wrote:
> 
> > > So if the current CPU can handle it, what is the problem?
> >   I hope this gets cleared out in my other email. But to make sure: If
> > other CPUs are idle (i.e. not appending to the printk buffer), we can well
> > handle the printing on the current CPU (with some breaks to allow
> > interrupts to be served etc.). If other CPUs are also appending to printk
> > buffer, that's where we really want to push the work to other CPUs.
> 
> I guess the question is, how does it migrate? I guess that's not so
> clear. Or do you just hope that the timer tick on another CPU will come
> in first and finish the job for you? As the list is global and all CPUs
> get to see it.
  There are two possibilities how the migration can happen:
1) a tick happens first on some other cpu and thus it will pick up the work
2) a printk happens first on some other cpu and thus it will pick printing
   in its console_unlock() call.

  I agree in both cases we depend on statistics, there's no real guarantee
the migration happens. But in my testing this seems to be enough.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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