From 3fbfbf7a3b66ec424042d909f14ba2ddf4372ea8 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 19 Aug 2012 21:35:53 -0700 Subject: rcu: Add callback-free CPUs RCU callback execution can add significant OS jitter and also can degrade both scheduling latency and, in asymmetric multiprocessors, energy efficiency. This commit therefore adds the ability for selected CPUs ("rcu_nocbs=" boot parameter) to have their callbacks offloaded to kthreads. If the "rcu_nocb_poll" boot parameter is also specified, these kthreads will do polling, removing the need for the offloaded CPUs to do wakeups. At least one CPU must be doing normal callback processing: currently CPU 0 cannot be selected as a no-CBs CPU. In addition, attempts to offline the last normal-CBs CPU will fail. This feature was inspired by Jim Houston's and Joe Korty's JRCU, and this commit includes fixes to problems located by Fengguang Wu's kbuild test robot. [ paulmck: Added gfp.h include file as suggested by Fengguang Wu. ] Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney --- Documentation/kernel-parameters.txt | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) (limited to 'Documentation') diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 9776f068306..9d2e5cb3a95 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -2394,6 +2394,27 @@ bytes respectively. Such letter suffixes can also be entirely omitted. ramdisk_size= [RAM] Sizes of RAM disks in kilobytes See Documentation/blockdev/ramdisk.txt. + rcu_nocbs= [KNL,BOOT] + In kernels built with CONFIG_RCU_NOCB_CPU=y, set + the specified list of CPUs to be no-callback CPUs. + Invocation of these CPUs' RCU callbacks will + be offloaded to "rcuoN" kthreads created for + that purpose. This reduces OS jitter on the + offloaded CPUs, which can be useful for HPC and + real-time workloads. It can also improve energy + efficiency for asymmetric multiprocessors. + + rcu_nocbs_poll [KNL,BOOT] + Rather than requiring that offloaded CPUs + (specified by rcu_nocbs= above) explicitly + awaken the corresponding "rcuoN" kthreads, + make these kthreads poll for callbacks. + This improves the real-time response for the + offloaded CPUs by relieving them of the need to + wake up the corresponding kthread, but degrades + energy efficiency by requiring that the kthreads + periodically wake up to do the polling. + rcutree.blimit= [KNL,BOOT] Set maximum number of finished RCU callbacks to process in one batch. -- cgit v1.2.3