linux-kernel - Re: [RFT][PATCH] sched, cgroup: Optimize load_balance

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Date:	Wed, 13 Jul 2011 17:46:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul Turner <pjt@...gle.com>, Nikhil Rao <ncrao@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>
Subject: Re: [RFT][PATCH] sched, cgroup: Optimize load_balance_fair()

On Wed, Jul 13, 2011 at 11:01:03PM +0200, Peter Zijlstra wrote:
> On Wed, 2011-07-13 at 10:13 -0700, Paul Turner wrote:
> > > +static void update_h_load(long cpu)
> > > +{
> > > +       walk_tg_tree(tg_load_down, tg_nop, (void *)cpu);
> > > +}
> > 
> > With a list_for_each_entry_reverse_rcu() this could also only operate
> > on the local hierarchy and avoid the tg tree walk. 
> 
> Ah, sadly that primitive cannot exist, rcu list primitives only keeps
> the fwd link.
> 
> Although I guess we could 'fix' that.

We could, at least in theory -- make list_del_rcu() not poison the
->prev link.  Or, given that there are use cases that absolutely cannot
tolerate following ->prev links, have a list_del_rcu_both() or something
so that list_del_rcu() keeps its current error checking.  Oddly enough,
__list_add_rcu() doesn't need to change because the rcu_assign_pointer()
for the predecessor's ->next pointer covers the successor's ->prev
pointer as well.  OK, a comment is clearly needed...

Of course, in a two-way-RCU doubly linked list, p->next->prev is not
necessarily equal to p.

But how deep/wide is the tree and how many cache misses are expected?
Would this solve a real problem?

							Thanx, Paul
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