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    SubjectRe: [PATCH tip/core/rcu 2/4] rcu: enable fourth level of TREE_RCU hierarchy
    On Wed, Dec 02, 2009 at 03:25:42PM -0800, Josh Triplett wrote:
    > On Wed, Dec 02, 2009 at 12:10:14PM -0800, Paul E. McKenney wrote:
    > > From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > >
    > > Enable a fourth level of rcu_node hierarchy for TREE_RCU and
    > > TREE_PREEMPT_RCU. This is for stress-testing and experiemental
    > > purposes only, although in theory this would enable 16,777,216 CPUs
    > > on 64-bit systems, though only 1,048,576 CPUs on 32-bit systems.
    > > Normal experimental use of this fourth level will normally set
    > > CONFIG_RCU_FANOUT=2, requiring a 16-CPU system, though the more
    > > adventurous (and more fortunate) experimenters may wish to chose
    > > CONFIG_RCU_FANOUT=3 for 81-CPU systems or even CONFIG_RCU_FANOUT=4 for
    > > 256-CPU systems.
    > >
    > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    >
    > I like this idea in general, but I have one suggestion on your boot-up
    > message:
    >
    > > --- a/kernel/rcutree.c
    > > +++ b/kernel/rcutree.c
    > [...]
    > > @@ -1877,6 +1878,9 @@ void __init rcu_init(void)
    > > #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
    > > printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
    > > #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
    > > +#if NUM_RCU_LVL_4 != 0
    > > + printk(KERN_INFO "Experimental four-level hierarchy is enabled.\n");
    > > +#endif /* #if NUM_RCU_LVL_4 != 0 */
    >
    > Rather than printing a message when people use the four-level hierarchy,
    > how about just printing a message any time someone has set
    > CONFIG_RCU_FANOUT manually rather than automatically, and including
    > NR_CPUS in that message? That should only occur when testing or when
    > trying to do NUMA optimization, and either way it seems worth noting.

    Good point! I will keep this patch as is, but I like the idea of having
    RCU note anything unusual at boot time, and so have added this to my
    todo list. Of course, if CONFIG_RCU_FANOUT starts getting set low as a
    matter of course for any (good) reason, then the definition of "anything
    unusual" would change, and hence the code would also change.

    > Either way the change seems fine to me. With or without that suggested
    > change:
    >
    > Acked-by: Josh Triplett <josh@joshtriplett.org>

    Thank you!

    Thanx, Paul


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