[PATCH 3/3] lib: support N as end of range in bitmap_parselist()

[Paul Gortmaker <paul.gortmaker@windriver.com>]

While this is done for all bitmaps, the original use case in mind was
for CPU masks and cpulist_parse().  Credit to Yury who suggested to
push it down from CPU subsys to bitmap - it simplified things a lot.

It seems that a common configuration is to use the 1st couple cores
for housekeeping tasks, and or driving a busy peripheral that generates
a lot of interrupts, or something similar.

This tends to leave the remaining ones to form a pool of similarly
configured cores to take on the real workload of interest to the user.

So on machine A - with 32 cores, it could be 0-3 for "system" and then
4-31 being used in boot args like nohz_full=, or rcu_nocbs= as part of
setting up the worker pool of CPUs.

But then newer machine B is added, and it has 48 cores, and so while
the 0-3 part remains unchanged, the pool setup cpu list becomes 4-47.

Deployment would be easier if we could just simply replace 31 and 47
with "N" and let the system substitute in the actual number at boot;
a number that it knows better than we do.

No need to have custom boot args per node, no need to do a trial boot
in order to snoop /proc/cpuinfo and/or /sys/devices/system/cpu - no
more fencepost errors of using 32 and 48 instead of 31 and 47.

Cc: Yury Norov <yury.norov@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 .../admin-guide/kernel-parameters.rst          |  4 ++++
 lib/bitmap.c                                   | 18 +++++++++++++-----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst
index 5e080080b058..668f0b69fb4f 100644
--- a/Documentation/admin-guide/kernel-parameters.rst
+++ b/Documentation/admin-guide/kernel-parameters.rst
@@ -68,6 +68,10 @@ For example one can add to the command line following parameter:
 
 where the final item represents CPUs 100,101,125,126,150,151,...
 
+The value "N" can be used as the end of a range, to represent the numerically
+last CPU on the system, i.e "foo_cpus=16-N" would be equivalent to "16-31" on
+a 32 core system.
+
 The following convenience aliases are also accepted and used:
 
         foo_cpus=all
diff --git a/lib/bitmap.c b/lib/bitmap.c
index a1010646fbe5..d498ea9d526b 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -571,7 +571,7 @@ static const char *bitmap_find_region_reverse(const char *start, const char *end
 	return end;
 }
 
-static const char *bitmap_parse_region(const char *str, struct region *r)
+static const char *bitmap_parse_region(const char *str, struct region *r, int nmaskbits)
 {
 	str = bitmap_getnum(str, &r->start);
 	if (IS_ERR(str))
@@ -583,9 +583,15 @@ static const char *bitmap_parse_region(const char *str, struct region *r)
 	if (*str != '-')
 		return ERR_PTR(-EINVAL);
 
-	str = bitmap_getnum(str + 1, &r->end);
-	if (IS_ERR(str))
-		return str;
+	str++;
+	if (*str == 'N') {
+		r->end = nmaskbits - 1;
+		str++;
+	} else {
+		str = bitmap_getnum(str, &r->end);
+		if (IS_ERR(str))
+			return str;
+	}
 
 	if (end_of_region(*str))
 		goto no_pattern;
@@ -628,6 +634,8 @@ static const char *bitmap_parse_region(const char *str, struct region *r)
  * Syntax: range:used_size/group_size
  * Example: 0-1023:2/256 ==> 0,1,256,257,512,513,768,769
  * Optionally the self-descriptive "all" or "none" can be used.
+ * The value 'N' can be used as the end of a range to indicate the maximum
+ * allowed value; i.e (nmaskbits - 1).
  *
  * Returns: 0 on success, -errno on invalid input strings. Error values:
  *
@@ -656,7 +664,7 @@ int bitmap_parselist(const char *buf, unsigned long *maskp, int nmaskbits)
 		if (buf == NULL)
 			return 0;
 
-		buf = bitmap_parse_region(buf, &r);
+		buf = bitmap_parse_region(buf, &r, nmaskbits);
 		if (IS_ERR(buf))
 			return PTR_ERR(buf);
 
-- 
2.17.1