[PATCH] Fix isocpus's param handling when CPUMASK_OFFSTACK=n.

[Rakib Mullick]

>  *On Fri, Oct 20, 2017 at 2:49 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>> Rakib Mullick <rakib.mullick@xxxxxxxxx> wrote:
>>  include/linux/cpumask.h | 16 ++++++++++++++++
>>  kernel/sched/topology.c |  8 +++++---
>>  2 files changed, 21 insertions(+), 3 deletions(-)
>
> What kernel is this against? It does not apply to the latest kernels.

It was against 4.14-rc4, prepared before -rc5 release. Please, consider
the below one, against -rc5.

 cpulist_parse() uses nr_cpumask_bits as limit to parse the
passed buffer from kernel commandline. What nr_cpumask_bits
represents varies depends upon CONFIG_CPUMASK_OFFSTACK option.
If CONFIG_CPUMASK_OFFSTACK=n, then nr_cpumask_bits is same as
NR_CPUS, which might not represent the # of cpus really exist
(default 64). So, there's a chance of gap between nr_cpu_ids
and NR_CPUS, which ultimately lead towards invalid cpulist_parse()
operation. For example, if isolcpus=9 is passed on a 8 cpu
system (CONFIG_CPUMASK_OFFSTACK=n) it doesn't show the error
that it suppose to.

This patch fixes this issue by effectively find out the last
cpu of the passed isolcpus list and checking it with nr_cpu_ids.
Also, fixes the error message where the nr_cpu_ids should be
nr_cpu_ids-1, since the cpu numbering starts from 0.

Signed-off-by: Rakib Mullick <rakib.mullick@xxxxxxxxx>
---
 include/linux/cpumask.h | 16 ++++++++++++++++
 kernel/sched/topology.c |  8 +++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index cd415b7..5631725 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -130,6 +130,11 @@ static inline unsigned int cpumask_first(const struct cpumask *srcp)
 	return 0;
 }
 
+static inline unsigned int cpumask_last(const struct cpumask *srcp)
+{
+	return 0;
+}
+
 /* Valid inputs for n are -1 and 0. */
 static inline unsigned int cpumask_next(int n, const struct cpumask *srcp)
 {
@@ -178,6 +183,17 @@ static inline unsigned int cpumask_first(const struct cpumask *srcp)
 	return find_first_bit(cpumask_bits(srcp), nr_cpumask_bits);
 }
 
+/**
+ * cpumask_last - get the last cpu in a cpumask
+ * @srcp:	- the cpumask pointer
+ *
+ * Returns	>= nr_cpumask_bits if no cpus set.
+ */
+static inline unsigned int cpumask_last(const struct cpumask *srcp)
+{
+	return find_last_bit(cpumask_bits(srcp), nr_cpumask_bits);
+}
+
 unsigned int cpumask_next(int n, const struct cpumask *srcp);
 
 /**
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index f1cf4f3..b9265c8 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -466,12 +466,14 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 /* Setup the mask of CPUs configured for isolated domains */
 static int __init isolated_cpu_setup(char *str)
 {
-	int ret;
+	int ret, lastcpu;
 
 	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);
+	lastcpu = cpumask_last(cpu_isolated_map);
+	if (ret || lastcpu >= nr_cpu_ids) {
+		pr_err("sched: Error, all isolcpus= values must be between 0 and %u\n",
+				nr_cpu_ids-1);
 		return 0;
 	}
 	return 1;
-- 
2.9.3