rcu: Allow rcu_nocbs= to specify all CPUs

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@linux.ibm.com>

1 files changed, 4 insertions, 1 deletions

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)
 {
 	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
-	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);