Messages in this thread Patch in this message | | | Date | Sat, 7 Mar 2015 13:52:23 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs |
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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" <paulmck@linux.vnet.ibm.com> > > > Signed-off-by: Alexander Gordeev <agordeev@redhat.com> > > > > 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 > agordeev@redhat.com >
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