[PATCH tip/core/rcu 09/18] rcu: Allow rcu_nocbs= to specify all CPUs

From: Paul E. McKenney
Date: Tue Mar 26 2019 - 19:14:27 EST

Currently, the rcu_nocbs= kernel boot parameter requires that a specific
list of CPUs be specified, and has no way to say "all of them".
As noted by user RavFX in a comment to Phoronix topic 1002538, this
is an inconvenient side effect of the removal of the RCU_NOCB_CPU_ALL
Kconfig option. This commit therefore enables the rcu_nocbs= kernel boot
parameter to be given the string "all", as in "rcu_nocbs=all" to specify
that all CPUs on the system are to have their RCU callbacks offloaded.

Another approach would be to make cpulist_parse() check for "all", but
there are uses of cpulist_parse() that do other checking, which could
conflict with an "all". This commit therefore focuses on the specific
use of cpulist_parse() in rcu_nocb_setup().

Just a note to other people who would like changes to Linux-kernel RCU:
If you send your requests to me directly, they might get fixed somewhat
faster. RavFX's comment was posted on January 22, 2018 and I first saw
it on March 5, 2019. And the only reason that I found it -at- -all- was
that I was looking for projects using RCU, and my search engine showed
me that Phoronix comment quite by accident. Your choice, though! ;-)

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
Documentation/admin-guide/kernel-parameters.txt | 4 +++-
kernel/rcu/tree_plugin.h | 5 ++++-
2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2b8ee90bb644..d377a2166b79 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3623,7 +3623,9 @@
see CONFIG_RAS_CEC help text.

rcu_nocbs= [KNL]
- The argument is a cpu list, as described above.
+ The argument is a cpu list, as described above,
+ except that the string "all" can be used to
+ specify every CPU on the system.

In kernels built with CONFIG_RCU_NOCB_CPU=y, set
the specified list of CPUs to be no-callback CPUs.
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d408661d5fb7..ed4a6dabf31d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1776,7 +1776,10 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp)
static int __init rcu_nocb_setup(char *str)
- cpulist_parse(str, rcu_nocb_mask);
+ if (!strcasecmp(str, "all"))
+ cpumask_setall(rcu_nocb_mask);
+ else
+ cpulist_parse(str, rcu_nocb_mask);
return 1;
__setup("rcu_nocbs=", rcu_nocb_setup);