linux-kernel - Re: Regression in 2.6.23-rc2-mm2, mounting cpusets causes a hang

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Message-ID: <20070815143125.GA11582@vino.hallyn.com>
Date:	Wed, 15 Aug 2007 09:31:25 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Lee Schermerhorn <Lee.Schermerhorn@...com>
Cc:	Christoph Lameter <clameter@....com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>, bob.picco@...com,
	nacc@...ibm.com, kamezawa.hiroyu@...fujitsu.com, mel@...net.ie,
	akpm@...ux-foundation.org, Balbir Singh <balbir@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	ckrm-tech <ckrm-tech@...ts.sourceforge.net>
Subject: Re: Regression in 2.6.23-rc2-mm2, mounting cpusets causes a hang

Quoting Lee Schermerhorn (Lee.Schermerhorn@...com):
> On Tue, 2007-08-14 at 14:56 -0700, Christoph Lameter wrote:
> > On Tue, 14 Aug 2007, Lee Schermerhorn wrote:
> > 
> > > > Ok then you did not have a NUMA system configured. So its okay for the 
> > > > dummies to ignore the stuff. CONFIG_NODES_SHIFT is a constant and does not 
> > > > change. The first bit is always set.
> > > 
> > > The first bit [node 0] is only set for the N_ONLINE [and N_POSSIBLE]
> > > mask.  We could add the static init for the other masks, but since
> > > non-numa platforms are going through the __build_all_zonelists, they
> > > might as well set the MEMORY bits explicitly.  Or, maybe you'll
> > > disagree ;-).
> > 
> > The bitmaps can be completely ignored if !NUMA.
> > 
> > In the non NUMA case we define
> > 
> > static inline int node_state(int node, enum node_states state)
> > {
> >         return node == 0;
> > }
> > 
> > So its always true for node 0. The "bit" is set.
> 
> The issue is with the N_*_MEMORY masks.  They don't get initialized
> properly because node_set_state() is a no-op if !NUMA.  So, where we
> look for intersections with or where we AND with the N_*_MEMORY masks we
> get the empty set.
> 
> > 
> > We are trying to get cpusets to work with !NUMA?
> 
> 
> Well, yes.  In Serge's case, he's trying to use cpusets with !NUMA.
> He'll have to comment on the reasons for that.  Looking at all of the

So I can lock a container to a cpu on a non-numa machine.

> #ifdefs and init/Kconfig, CPUSET does not depend on NUMA--only SMP and
> CONTAINERS [altho' methinks CPUSET should select CONTAINERS rather than
> depend on it...].  So, you can use cpusets to partition of cpus in
> non-NUMA configs.
> 
> In the more general case, tho', I'm looking at all uses of the
> node_online_map and for_each_online_node, for instances where they
> should be replaced with one of the *_MEMORY masks.  IMO, generic code
> that is compiled independent of any CONFIG option, like NUMA,  should
> just work, independent of the config.  Currently, as Serge has shown,
> this is not the case.  So, I think we should fix the *_MEMORY maps to be
> correctly populated in both the NUMA and !NUMA cases.  A couple of
> options:
> 
> 1) just use node_set() when populating the masks,
> 
> 2) initialize all masks to include at least cpu/node 0 in the !NUMA
> case.
> 
> Serge chose #1 to fix his problem.  I followed his lead to fix the other
> 2 places where node_set_state() was being used to initialize the NORMAL
> memory node mask and the CPU node mask.  This will add a few unnecessary
> instructions to !NUMA configs, so we could change to #2.
> 
> Thoughts?

Paul, is the mems stuff in cpusets only really useful for NUMA cases?
(I think it is... but am not sure)  If so I suppose one alternative
could be to just disable that when !NUMA.  But disabling cpusets when
!NUMA is completely wrong.

I personally would think that 1) is still the best option.  Otherwise
the action

	echo $SOME_CPU > /cpusets/set1/cpu
	echo $SOME_CPU > /cpusets/set1/mems

works on a numa machine, and is wrong on a non-numa machine.  With
option 1, the second part doesn't actually restrict the memory, but
at least /cpusets/set1/mems exists and $SOME_CPU doesn't have to be 0 to
be valid.

-serge
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