linux-kernel - Re: CONFIG_NO_HZ_FULL + CONFIG_PREEMPT_RT

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Date:	Thu, 7 Nov 2013 14:13:02 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Mike Galbraith <bitbucket@...ine.de>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: CONFIG_NO_HZ_FULL + CONFIG_PREEMPT_RT_FULL = nogo

On Thu, 7 Nov 2013, Frederic Weisbecker wrote:
> On Thu, Nov 07, 2013 at 12:21:11PM +0100, Thomas Gleixner wrote:
> > Though it's not a full solution. It needs some thought versus the
> > softirq code of timers. Assume we have only one timer queued 1000
> > ticks into the future. So this change will cause the timer softirq not
> > to be called until that timer expires and then the timer softirq is
> > going to do 1000 loops until it catches up with jiffies. That's
> > anything but pretty ...
> 
> I see, so the problem is that we raise the timer softirq unconditionally
> from the tick?

Right.
 
> Ok we definetly don't want to keep that behaviour, even if softirqs are not
> threaded, that's an overhead. So I'm looking at that loop in __run_timers()
> and I guess you mean the "base->timer_jiffies" incrementation?
>
> That's indeed not pretty. How do we handle exit from long dynticks
> idle periods? Are we doing that loop until we catch up with the new
> jiffies?

Right. I realized that right after I hit send :)
 
> Then it relies on the timer cascade stuff which is very obscure code to me...

It's not that bad, really. I have an idea how to fix that. Needs some
rewriting though.

Thanks,

	tglx

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