linux-kernel - Re: [PATCH 3/5] irq_work: Implement remote queueing

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Message-ID: <20140603150819.GC23860@localhost.localdomain>
Date:	Tue, 3 Jun 2014 17:08:21 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 3/5] irq_work: Implement remote queueing

On Tue, Jun 03, 2014 at 05:00:08PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 03, 2014 at 04:40:18PM +0200, Frederic Weisbecker wrote:
> > irq work currently only supports local callbacks. However its code
> > is mostly ready to run remote callbacks and we have some potential user.
> > 
> > The full nohz subsystem currently open codes its own remote irq work
> > on top of the scheduler ipi when it wants a CPU to reevaluate its next
> > tick. However this ad hoc solution bloats the scheduler IPI.
> > 
> > Lets just extend the irq work subsystem to support remote queuing on top
> > of the generic SMP IPI to handle this kind of user. This shouldn't add
> > noticeable overhead.
> > 
> > Suggested-by: Peter Zijlstra <peterz@...radead.org>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Kevin Hilman <khilman@...aro.org>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Acked-by: Peter Zijlstra <peterz@...radead.org>
> 
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Viresh Kumar <viresh.kumar@...aro.org>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > ---
> >  include/linux/irq_work.h |  2 ++
> >  kernel/irq_work.c        | 22 +++++++++++++++++++++-
> >  kernel/smp.c             |  4 ++++
> >  3 files changed, 27 insertions(+), 1 deletion(-)
> 
> > @@ -198,6 +199,9 @@ void generic_smp_call_function_single_interrupt(void)
> >  		csd->func(csd->info);
> >  		csd_unlock(csd);
> >  	}
> > +
> > +	/* Handle irq works queued remotely by irq_work_queue_on() */
> > +	irq_work_run();
> >  }
> 
> One could possibly extend that comment by stating that we explicitly run
> the irq_work bits after the function bits, because the function bits are
> typically synchronous and have people waiting on them, while not so for
> the irq_works.
> 

Good point! I'll expand the comment.
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