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doc: Document rcutree.nocb_nobypass_lim_per_jiffy kernel parameter

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This commit provides documentation for the kernel parameter controlling
RCU's handling of callback floods on offloaded (rcu_nocbs) CPUs.
This parameter might be obscure, but it is always there when you need it.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Reported-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
This commit is contained in:
Paul E. McKenney 2022-04-27 09:24:31 -07:00
parent 71de1e34f1
commit 89f7f29140
1 changed files with 13 additions and 0 deletions
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Documentation/admin-guide/kernel-parameters.txt
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@ -4681,6 +4681,19 @@
invoked in the context of an rcuoc kthread, which
scheduler will preempt as it does any other task.
rcutree.nocb_nobypass_lim_per_jiffy= [KNL]
On callback-offloaded (rcu_nocbs) CPUs,
RCU reduces the lock contention that would
otherwise be caused by callback floods through
use of the ->nocb_bypass list. However, in the
common non-flooded case, RCU queues directly to
the main ->cblist in order to avoid the extra
overhead of the ->nocb_bypass list and its lock.
But if there are too many callbacks queued during
a single jiffy, RCU pre-queues the callbacks into
the ->nocb_bypass queue. The definition of "too
many" is supplied by this kernel boot parameter.
rcutree.rcu_nocb_gp_stride= [KNL]
Set the number of NOCB callback kthreads in
each group, which defaults to the square root