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rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

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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 <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2017-02-03 09:27:00 -08:00
parent 468d01bec5
commit 0248288009
1 changed files with 8 additions and 2 deletions
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10
init/Kconfig
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@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF
initialization. These systems tend to run CPU-bound, and thus initialization. These systems tend to run CPU-bound, and thus
are not helped by synchronized interrupts, and thus tend to are not helped by synchronized interrupts, and thus tend to
skew them, which reduces lock contention enough that large 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.
Select a specific number if testing RCU itself. Select a specific number if testing RCU itself.
Select the maximum permissible value for large systems. Select the maximum permissible value for large systems, but
please understand that you may also need to set the skew_tick
kernel boot parameter to avoid contention on the rcu_node
structure's locks.
Take the default if unsure. Take the default if unsure.