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rcu/nocb: Allow empty "rcu_nocbs" kernel parameter 

Allow the rcu_nocbs kernel parameter to be specified just by itself,
without specifying any CPUs.  This allows systems administrators to use
"rcu_nocbs" to specify that none of the CPUs are to be offloaded at boot
time, but than any of them may be offloaded at runtime via cpusets.

In contrast, if the "rcu_nocbs" or "nohz_full" kernel parameters are not
specified at all, then not only are none of the CPUs offloaded at boot,
none of them can be offloaded at runtime, either.

While in the area, modernize the description of the "rcuo" kthreads'
naming scheme.

Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2021-11-23 01:37:07 +01:00 committed by Paul E. McKenney
parent 2cf4528d6d
commit d2cf0854d7
2 changed files with 29 additions and 16 deletions
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35
Documentation/admin-guide/kernel-parameters.txt
View File

@ -4343,19 +4343,30 @@
Disable the Correctable Errors Collector,
see CONFIG_RAS_CEC help text.
rcu_nocbs= [KNL]
The argument is a cpu list, as described above.
rcu_nocbs[=cpu-list]
[KNL] The optional argument is a cpu list,
as described above.
In kernels built with CONFIG_RCU_NOCB_CPU=y, set
the specified list of CPUs to be no-callback CPUs.
Invocation of these CPUs' RCU callbacks will be
offloaded to "rcuox/N" kthreads created for that
purpose, where "x" is "p" for RCU-preempt, and
"s" for RCU-sched, and "N" is the CPU number.
This reduces OS jitter on the offloaded CPUs,
which can be useful for HPC and real-time
workloads. It can also improve energy efficiency
for asymmetric multiprocessors.
In kernels built with CONFIG_RCU_NOCB_CPU=y,
enable the no-callback CPU mode, which prevents
such CPUs' callbacks from being invoked in
softirq context. Invocation of such CPUs' RCU
callbacks will instead be offloaded to "rcuox/N"
kthreads created for that purpose, where "x" is
"p" for RCU-preempt, "s" for RCU-sched, and "g"
for the kthreads that mediate grace periods; and
"N" is the CPU number. This reduces OS jitter on
the offloaded CPUs, which can be useful for HPC
and real-time workloads. It can also improve
energy efficiency for asymmetric multiprocessors.
If a cpulist is passed as an argument, the specified
list of CPUs is set to no-callback mode from boot.
Otherwise, if the '=' sign and the cpulist
arguments are omitted, no CPU will be set to
no-callback mode from boot but the mode may be
toggled at runtime via cpusets.
rcu_nocb_poll [KNL]
Rather than requiring that offloaded CPUs

10
kernel/rcu/tree_nocb.h
View File

@ -66,14 +66,16 @@ static bool rcu_nocb_is_setup;
static int __init rcu_nocb_setup(char *str)
{
alloc_bootmem_cpumask_var(&rcu_nocb_mask);
if (cpulist_parse(str, rcu_nocb_mask)) {
pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
cpumask_setall(rcu_nocb_mask);
if (*str == '=') {
if (cpulist_parse(++str, rcu_nocb_mask)) {
pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
cpumask_setall(rcu_nocb_mask);
}
}
rcu_nocb_is_setup = true;
return 1;
}
__setup("rcu_nocbs=", rcu_nocb_setup);
__setup("rcu_nocbs", rcu_nocb_setup);
static int __init parse_rcu_nocb_poll(char *arg)
{