Message-ID: <20150701184538.GN3717@linux.vnet.ibm.com>
Date:	Wed, 1 Jul 2015 11:45:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, tj@...nel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, der.herr@...r.at, dave@...olabs.net,
	riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

On Wed, Jul 01, 2015 at 06:16:40PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 01, 2015 at 08:56:55AM -0700, Paul E. McKenney wrote:
> > On Wed, Jul 01, 2015 at 01:56:42PM +0200, Peter Zijlstra wrote:
> > Odd that you have four of eight of the rcuos CPUs with higher consumption
> > than the others.  I would expect three of eight.  Are you by chance running
> > an eight-core system with hyperthreading disabled in hardware, via boot
> > parameter, or via explicit offline?  The real question I have is "is
> > nr_cpu_ids equal to 16 rather than to 8?"
> 
> It should not, but I'd have to instrument to be sure. Its a regular
> 4 core + ht part.
> 
> model name      : Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz

Well, if nr_cpu_ids is equal to 8, I likely need to recheck my math.

> > Also, do you have nohz_full set?
> 
> Nope..
> 
> > Just wondering why callback offloading
> > is enabled.  (If you want it enabled, fine, but from what I can see your
> > workload isn't being helped by it and it does have higher overhead.)
> 
> I think this is a distro .config; every time I strip the desktop kernel
> I end up needing a driver I hadn't built. Clearly I've not really paid
> attention to the RCU options.

OK, early versions of RHEL definitely would do what you have by default,
and I would need to check with Rik to find out what stuff got backported
when.

> > Even if you don't want offloading and do disable it, it would be good to
> > reduce the penalty.  Is there something I can do to reduce the overhead
> > of waking several kthreads?  Right now, I just do a series of wake_up()
> > calls, one for each leader rcuos kthread.
> > 
> > Oh, are you running v3.10 or some such?  If so, there are some more
> > recent RCU changes that can help with this.  They are called out here:
> 
> Not that old, but not something recent either. I'll upgrade and see if
> it goes away. I really detest rebooting the desktop, but it needs to
> happen every so often.

Feel free to send me the .config, the exact version, and any boot
parameters you have.  That would allow me to tell you whether or not
moving ahead would do you any good.

> > > Yah, if only we could account it back to whomever caused it :/
> > 
> > It could be done, but would require increasing the size of rcu_head.
> > And would require costly fine-grained timing of callback execution.
> > Not something for production systems, I would guess.
> 
> Nope :/ I know.
> 
> > > What I was talking about was the interaction between the force
> > > quiescence state and the poking detectoring that a QS had indeed be
> > > started.
> > 
> > It gets worse.
> > 
> > Suppose that a grace period is already in progess.  You cannot leverage
> > its use of the combining tree because some of the CPUs might have already
> > indicated a quiescent state, which means that the current grace period
> > won't necessarily wait for all of the CPUs that the concurrent expedited
> > grace period needs to wait on.  So you need to kick the current grace
> > period, wait for it to complete, wait for the next one to start (with
> > all the fun and exciting issues called out earlier), do the expedited
> > grace period, then wait for completion.
> 
> Ah yes. You do do find the fun cases :-)

Given that I am RCU maintainer, I had better be able to.  A large
quantity of them rushed into my head when you suggested this, hence my
initial reaction.  That said, Oleg is probably better at finding fun
cases than am I.

> > > If you wake it unconditionally, even if there's nothing to do, then yes
> > > that'd be a waste of cycles.
> > 
> > Heh!  You are already complaining about rcu_sched consuming 0.7%
> > of your system, and rightfully so.  Increasing this overhead still
> > further therefore cannot be considered a good thing unless there is some
> > overwhelming benefit.  And I am not seeing that benefit.  Perhaps due
> > to a failure of imagination, but until someone enlightens me, I have to
> > throttle the wakeups -- or, perhaps better, omit the wakeups entirely.
> > 
> > Actually, I am not convinced that I should push any of the patches that
> > leverage expedited grace periods to help out normal grace periods.
> 
> It would seem a shame not to.. I've not yet had time to form a coherent
> reply to that thread though.

Well, it does increase complexity and coupling, and I don't see that
it provides big-animal benefits to justify this.  Again, might be just
insufficient imagination on my part, but...

							Thanx, Paul

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