linux-kernel - Re: [PATCH 09/32] nohz: Move ts->idle

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Message-ID: <20110830144500.GR9748@somewhere.redhat.com>
Date:	Tue, 30 Aug 2011 16:45:02 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 09/32] nohz: Move ts->idle_calls into strict idle logic

On Mon, Aug 29, 2011 at 08:33:23PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-29 at 20:23 +0200, Frederic Weisbecker wrote:
> 
> > > Well, no, on interrupt return you shouldn't do anything. If you've
> > > stopped the tick it stays stopped until you do something that needs it,
> > > then that action will re-enable it.
> > 
> > Sure, when something needs the tick in this mode, we usually
> > receive an IPI and restart the tick from there but then
> > tick_nohz_stop_sched_tick() handles the cases with *needs_cpu()
> > very well on interrupt return (our IPI return) by doing a kind
> > of "light" HZ mode by logically switching to nohz mode but
> > with the next timer happening in HZ, assuming it's a matter
> > of one tick and we will switch to a real nohz behaviour soon.
> > 
> > I don't see a good reason to duplicate that logic with a pure
> > restart from the IPI.
> 
> That sounds like an optimization, and should thus be done later.

The optimization is already there upstream. I can split the logic for
non-idle case but I'm not sure about the point of that.
 
> > > > That said I wonder if some of the above conditions should restore a periodic
> > > > behaviour on interrupt return...
> > > 
> > > I would expect the tick not to be stopped when tick_nohz_can_stop_tick()
> > > returns false. If it returns true, then I expect anything that needs it
> > > to re-enable it.
> > > 
> > 
> > Yeah. In the case of need_resched() in idle I believe the CPU doesn't
> > really go to sleep later so it should be fine. But for the case of
> > softirq pending or nohz_mode, I'm not sure...
> 
> softirqs shouldn't be pending when you go into nohz mode..

You mean it can't happen or we don't want that to happen?

> 
> That is, I'm really not seeing what's wrong with the very simple:
> 
> 
>   if (tick_nohz_can_stop_tick())
> 	tick_nohz_stop_tick();
> 
> 
> and relying on everybody who invalidates tick_nohz_can_stop_tick(), to
> do:
> 
>   tick_nohz_start_tick();

May be for the non-idle case. But for the idle case I need to ensure
this is necessary somewhere.

> 
> I'm also not quite sure why you always IPI, is that to avoid lock
> inversions?

Exactly! I think I wrote that to some changelog but I'm not sure. I'll
check that.
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