linux-kernel - Re: [RFC PATCH 10/15] nohz_task: Enter in extended quiescent state when in userspace

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Date:	Tue, 21 Dec 2010 15:06:40 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 10/15] nohz_task: Enter in extended quiescent state
 when in userspace

On Tue, Dec 21, 2010 at 09:04:54AM +0100, Peter Zijlstra wrote:
> On Tue, 2010-12-21 at 02:27 +0100, Frederic Weisbecker wrote:
> > On Mon, Dec 20, 2010 at 05:18:40PM +0100, Peter Zijlstra wrote:
> > > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> > > > (we check if the local cpu is in nohz mode, which means
> > > > no other task compete on that CPU)
> > > 
> > > You keep repeating that definition, but its not true.. It means there is
> > > not work for the tick to do, the tick does _tons_ more besides
> > > preemption, so nr_running==1 is necessary but not sufficient. 
> > 
> > Sure but the point is that if the tick is not running, it means
> > that the nohz task is the only task running on that CPU.
> 
> No, that too isn't true, the cpu could be idle.
> 
> > Now indeed there are other reasons for the tick to restart like RCU
> > or the effective nohz mode to physically happen or to be delayed,
> > which is decided by tick_nohz_stop_sched_tick().
> 
> You really really really badly need to read through the whole tick path
> and look at all the things it does, put them in a list, then look at the
> current nohz code, mark those it deals with, then go through the nohz
> code again and find the nr_running==0 assumptions, then make sure you've
> covered everything.
> 
> I'm very confident you'll find a number of things to fix. At the very
> least your current patch set totally forgets about task runtime
> accounting (account_process_tick()), the existing NO_HZ doesn't need to
> worry about that because the system is idle so all it needs to do is add
> idle ticks when it wakes up (and possibly steal time for the virt muck).
> 
> You also miss the profile_tick(), and you need to go through the load
> accounting muck (both of them) to see if there are any nr_running==0
> assumptions there.
> 
> You also need to fix the perf counter list rotation stuff, and again,
> check if no_hz load-balancing can deal with the nr_running==1 situation.
> 
> Now, there might be more, this is just a quick one-minute scan through
> the tick code, but the fact that nowhere in this whole patch-set is even
> a mention of these things makes me worry about your whole approach.


Yeah it's right I missed a lot of important things. I'll try to handle
them for the next take.
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