On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > If you set RCU_FANOUT_LEAF too high, you can get lock contention > on the leaf rcu_node, and you should boot with the skew_tick kernel > parameter set in order to avoid this lock contention. This commit > therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state > this. > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > --- > init/Kconfig | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/init/Kconfig b/init/Kconfig > index a92f27da4a27..946e561e67b7 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF > initialization. These systems tend to run CPU-bound, and thus > are not helped by synchronized interrupts, and thus tend to > skew them, which reduces lock contention enough that large > - leaf-level fanouts work well. > + leaf-level fanouts work well. That said, setting leaf-level > + fanout to a large number will likely cause problematic > + lock contention on the leaf-level rcu_node structures unless > + you boot with the skew_tick kernel parameter.
Why mention a way out of a problem you shouldn't have to begin with? Just state its bad and result in lock contention and leave it at that.
