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rcu: Optionally run grace-period kthreads at real-time priority

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Recent testing has shown that under heavy load, running RCU's grace-period
kthreads at real-time priority can improve performance (according to 0day
test robot) and reduce the incidence of RCU CPU stall warnings.  However,
most systems do just fine with the default non-realtime priorities for
these kthreads, and it does not make sense to expose the entire user
base to any risk stemming from this change, given that this change is
of use only to a few users running extremely heavy workloads.

Therefore, this commit allows users to specify realtime priorities
for the grace-period kthreads, but leaves them running SCHED_OTHER
by default.  The realtime priority may be specified at build time
via the RCU_KTHREAD_PRIO Kconfig parameter, or at boot time via the
rcutree.kthread_prio parameter.  Either way, 0 says to continue the
default SCHED_OTHER behavior and values from 1-99 specify that priority
of SCHED_FIFO behavior.  Note that a value of 0 is not permitted when
the RCU_BOOST Kconfig parameter is specified.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney
2014-12-12 07:37:48 -08:00
parent e3663b1024
commit a94844b22a
3 changed files with 27 additions and 8 deletions
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init/Kconfig
+4 -3
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@@ -668,9 +668,10 @@ config RCU_BOOST
config RCU_KTHREAD_PRIO
int "Real-time priority to use for RCU worker threads"
range 1 99
depends on RCU_BOOST
default 1
range 1 99 if RCU_BOOST
range 0 99 if !RCU_BOOST
default 1 if RCU_BOOST
default 0 if !RCU_BOOST
help
This option specifies the SCHED_FIFO priority value that will be
assigned to the rcuc/n and rcub/n threads and is also the value