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authorPaul E. McKenney <paul.mckenney@linaro.org>2012-06-26 20:45:57 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2012-09-23 07:41:54 -0700
commitd40011f601b450396104de42c631981502946cf0 (patch)
tree62205b755bca9391c6924764387d4417e91014d3 /Documentation/kernel-parameters.txt
parent394f2769aa0dbcf027bae6fb52835e25e05d332e (diff)
rcu: Control grace-period duration from sysfs
Although almost everyone is well-served by the defaults, some uses of RCU benefit from shorter grace periods, while others benefit more from the greater efficiency provided by longer grace periods. Situations requiring a large number of grace periods to elapse (and wireshark startup has been called out as an example of this) are helped by lower-latency grace periods. Furthermore, in some embedded applications, people are willing to accept a small degradation in update efficiency (due to there being more of the shorter grace-period operations) in order to gain the lower latency. In contrast, those few systems with thousands of CPUs need longer grace periods because the CPU overhead of a grace period rises roughly linearly with the number of CPUs. Such systems normally do not make much use of facilities that require large numbers of grace periods to elapse, so this is a good tradeoff. Therefore, this commit allows the durations to be controlled from sysfs. There are two sysfs parameters, one named "jiffies_till_first_fqs" that specifies the delay in jiffies from the end of grace-period initialization until the first attempt to force quiescent states, and the other named "jiffies_till_next_fqs" that specifies the delay (again in jiffies) between subsequent attempts to force quiescent states. They both default to three jiffies, which is compatible with the old hard-coded behavior. At some future time, it may be possible to automatically increase the grace-period length with the number of CPUs, but we do not yet have sufficient data to do a good job. Preliminary data indicates that we should add an addiitonal jiffy to each of the delays for every 200 CPUs in the system, but more experimentation is needed. For now, the number of systems with more than 1,000 CPUs is small enough that this can be relegated to boot-time hand tuning. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Diffstat (limited to 'Documentation/kernel-parameters.txt')
-rw-r--r--Documentation/kernel-parameters.txt11
1 files changed, 11 insertions, 0 deletions
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index ad7e2e5088c..55ada0471f9 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2385,6 +2385,17 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
rcutree.rcu_cpu_stall_timeout= [KNL,BOOT]
Set timeout for RCU CPU stall warning messages.
+ rcutree.jiffies_till_first_fqs= [KNL,BOOT]
+ Set delay from grace-period initialization to
+ first attempt to force quiescent states.
+ Units are jiffies, minimum value is zero,
+ and maximum value is HZ.
+
+ rcutree.jiffies_till_next_fqs= [KNL,BOOT]
+ Set delay between subsequent attempts to force
+ quiescent states. Units are jiffies, minimum
+ value is one, and maximum value is HZ.
+
rcutorture.fqs_duration= [KNL,BOOT]
Set duration of force_quiescent_state bursts.