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Description of problem:

Although rcu no callbacks is useful for the majority of -rt workloads. There are cases where this feature can cause bad impact in the -rt performance.

For example, if the rcuc/ thread needs to wake up many offload threads, for instance in a large box with dozens CPUs, this can cause an interface that can be higher than the 150 microseconds we guarantee in the -rt products.

As we can enable the rcu no callbacks via kernel command line, we still can have the same behavior of CONFIG_RCU_NOCB_CPU_ALL=y by using the rcu_nocbs=<allcpus>. So it is safe to turn it off. In fact, this gives us more flexibility than have all CPUs rcu_nocbs by default

This change was made in:

kernel-rt-3.10.0-439.rt56.327.el7

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2016-2584.html