sched/isolcpus: Fix "isolcpus=" boot parameter handling when !CONFIG_CPUMASK_OFFSTACK

cpulist_parse() uses nr_cpumask_bits as a limit to parse the
passed buffer from kernel commandline. What nr_cpumask_bits
represents varies depending upon the CONFIG_CPUMASK_OFFSTACK option:

 - If CONFIG_CPUMASK_OFFSTACK=n, then nr_cpumask_bits is the same as
   NR_CPUS, which might not represent the # of CPUs that really exist
   (default 64). So, there's a chance of a gap between nr_cpu_ids
   and NR_CPUS, which ultimately lead towards invalid cpulist_parse()
   operation. For example, if isolcpus=9 is passed on an 8 cpu
   system (CONFIG_CPUMASK_OFFSTACK=n) it doesn't show the error
   that it's supposed to.

This patch fixes this bug by finding the last CPU of the passed
isolcpus= list and checking it against nr_cpu_ids.

It also fixes the error message where the nr_cpu_ids should be
nr_cpu_ids-1, since CPU numbering starts from 0.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: adobriyan@gmail.com
Cc: akpm@linux-foundation.org
Cc: longman@redhat.com
Cc: mka@chromium.org
Cc: tj@kernel.org
Link: http://lkml.kernel.org/r/20171023130154.9050-1-rakib.mullick@gmail.com
[ Enhanced the changelog and the kernel message. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

 include/linux/cpumask.h |   16 ++++++++++++++++
 kernel/sched/topology.c |    4 ++--
 2 files changed, 18 insertions(+), 2 deletions(-)

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index e50450c2fed8..e3d31b0880dc 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -476,8 +476,8 @@ static int __init isolated_cpu_setup(char *str)
 
 	alloc_bootmem_cpumask_var(&cpu_isolated_map);
 	ret = cpulist_parse(str, cpu_isolated_map);
-	if (ret) {
-		pr_err("sched: Error, all isolcpus= values must be between 0 and %u\n", nr_cpu_ids);
+	if (ret || cpumask_last(cpu_isolated_map) >= nr_cpu_ids) {
+		pr_err("sched: Error, all isolcpus= values must be between 0 and %u - ignoring them.\n", nr_cpu_ids-1);
 		return 0;
 	}
 	return 1;