On Sat, Mar 07, 2015 at 06:48:21PM +0000, Alexander Gordeev wrote:
> On Sat, Mar 07, 2015 at 09:42:34AM -0800, Paul E. McKenney wrote:
> > On Sat, Mar 07, 2015 at 06:03:36PM +0100, Alexander Gordeev wrote:
> > > Currently a condition when RCU tree is unable to accommodate
> > > the configured number of CPUs is not permitted and causes
> > > a fall back to compile-time values. However, the code has no
> > > means to exceed the RCU tree capacity neither at compile-time
> > > nor in run-time. Therefore, if the condition is met in run-
> > > time then it indicates a serios problem elsewhere and should
> > > be handled with a panic.
> > >
> > > Cc: "Paul E. McKenney" <[email protected]>
> > > Signed-off-by: Alexander Gordeev <[email protected]>
> >
> > The place to put a check like this is in the code that calculates
> > nr_cpu_ids. And at least some (perhaps all) are set up so that nr_cpu_ids
> > cannot exceed NR_CPUS, which would render this check redundant.
>
> The emphasis here the existing check (... n > rcu_capacity[MAX_RCU_LVLS])
> (below as [1]) should not cause the fall back to compiled-time values.
> It either must panic or, as you say - redundant.
You are right, I responded too early on a Saturday. The point of the
check below is indeed to verify that RCU's calculations are correct.
So do the testing with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF both
equal to five, and rebase to the rcu/next branch of:
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
And I will give them a spin.
Thanx, Paul
> > So I have to say "no" to this one.
> >
> > Thanx, Paul
> >
> > > ---
> > > kernel/rcu/tree.c | 15 +++++++++------
> > > 1 file changed, 9 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 48d640c..7588c7f 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3889,16 +3889,19 @@ static void __init rcu_init_geometry(void)
> > > rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
> > >
> > > /*
> > > + * The tree must be able to accommodate the configured number of CPUs.
> > > + * If this limit is exceeded than we have a serious problem elsewhere.
> > > + *
> > > * The boot-time rcu_fanout_leaf parameter is only permitted
> > > * to increase the leaf-level fanout, not decrease it. Of course,
> > > * the leaf-level fanout cannot exceed the number of bits in
> > > - * the rcu_node masks. Finally, the tree must be able to accommodate
> > > - * the configured number of CPUs. Complain and fall back to the
> > > - * compile-time values if these limits are exceeded.
> > > + * the rcu_node masks. Complain and fall back to the compile-
> > > + * time values if these limits are exceeded.
> > > */
> > > - if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> > > - rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
> > > - n > rcu_capacity[MAX_RCU_LVLS]) {
>
> [1]
>
> > > + if (n > rcu_capacity[MAX_RCU_LVLS])
> > > + panic("rcu_init_geometry: rcu_capacity[] is too small");
> > > + else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> > > + rcu_fanout_leaf > sizeof(unsigned long) * 8) {
> > > WARN_ON(1);
> > > return;
> > > }
> > > --
> > > 1.8.3.1
> > >
> >
>
> --
> Regards,
> Alexander Gordeev
> [email protected]
>
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