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rcu: Add rcu_normal kernel parameter to suppress expediting

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Although expedited grace periods can be quite useful, and although their
OS jitter has been greatly reduced, they can still pose problems for
extreme real-time workloads.  This commit therefore adds a rcu_normal
kernel boot parameter (which can also be manipulated via sysfs)
to suppress expedited grace periods, that is, to treat requests for
expedited grace periods as if they were requests for normal grace periods.
If both rcu_expedited and rcu_normal are specified, rcu_normal wins.
This means that if you are relying on expedited grace periods to speed up
boot, you will want to specify rcu_expedited on the kernel command line,
and then specify rcu_normal via sysfs once boot completes.

Signed-off-by: Paul E. McKenney <[email protected]>
This commit is contained in:
Paul E. McKenney 2015-11-24 15:44:06 -08:00
commit 5a9be7c628
7 changed files with 65 additions and 8 deletions
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19
Documentation/kernel-parameters.txt
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@ -3296,6 +3296,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
rcutorture.verbose= [KNL]
Enable additional printk() statements.
rcupdate.rcu_cpu_stall_suppress= [KNL]
Suppress RCU CPU stall warning messages.
rcupdate.rcu_cpu_stall_timeout= [KNL]
Set timeout for RCU CPU stall warning messages.
rcupdate.rcu_expedited= [KNL]
Use expedited grace-period primitives, for
example, synchronize_rcu_expedited() instead
@ -3303,11 +3309,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
but can increase CPU utilization, degrade
real-time latency, and degrade energy efficiency.
rcupdate.rcu_cpu_stall_suppress= [KNL]
Suppress RCU CPU stall warning messages.
rcupdate.rcu_cpu_stall_timeout= [KNL]
Set timeout for RCU CPU stall warning messages.
rcupdate.rcu_normal= [KNL]
Use only normal grace-period primitives,
for example, synchronize_rcu() instead of
synchronize_rcu_expedited(). This improves
real-time latency, CPU utilization, and energy
efficiency, but can expose users to increased
grace-period latency. This parameter overrides
rcupdate.rcu_expedited.
rcupdate.rcu_task_stall_timeout= [KNL]
Set timeout in jiffies for RCU task stall warning