[PATCH 6/8] lib: bitmap: support "N" as an alias for size of bitmap

[Paul Gortmaker]

While this is done for all bitmaps, the original use case in mind was
for CPU masks and cpulist_parse() as described below.

It seems that a common configuration is to use the 1st couple cores for
housekeeping tasks.  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.

Multiple deployment becomes easier when we can 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.

Cc: Yury Norov <yury.norov@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Suggested-by: Yury Norov <yury.norov@xxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.rst |  2 ++
 lib/bitmap.c                                    | 12 ++++++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst
index 682ab28b5c94..850917f19476 100644
--- a/Documentation/admin-guide/kernel-parameters.rst
+++ b/Documentation/admin-guide/kernel-parameters.rst
@@ -68,6 +68,8 @@ 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 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.
 
 
 This document may not be entirely up to date and comprehensive. The command
diff --git a/lib/bitmap.c b/lib/bitmap.c
index f65be2f148fd..2fdd00b312c3 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -519,11 +519,17 @@ static int bitmap_check_region(const struct region *r)
 	return 0;
 }
 
-static const char *bitmap_getnum(const char *str, unsigned int *num)
+static const char *__bitmap_getnum(const char *str, unsigned int nbits,
+				    unsigned int *num)
 {
 	unsigned long long n;
 	unsigned int len;
 
+	if (str[0] == 'N') {
+		*num = nbits - 1;
+		return str + 1;
+	}
+
 	len = _parse_integer(str, 10, &n);
 	if (!len)
 		return ERR_PTR(-EINVAL);
@@ -533,7 +539,7 @@ static const char *bitmap_getnum(const char *str, unsigned int *num)
 	*num = n;
 	return str + len;
 }
-#define bitmap_getrnum(s, r, pos) bitmap_getnum(s, &(r->pos))
+#define bitmap_getrnum(s, r, pos) __bitmap_getnum(s, r->nbits, &(r->pos))
 
 static inline bool end_of_str(char c)
 {
@@ -626,6 +632,8 @@ static const char *bitmap_parse_region(const char *str, struct region *r)
  * From each group will be used only defined amount of bits.
  * Syntax: range:used_size/group_size
  * Example: 0-1023:2/256 ==> 0,1,256,257,512,513,768,769
+ * The value 'N' can be used as a dynamically substituted token for the
+ * maximum allowed value; i.e (nmaskbits - 1).
  *
  * Returns: 0 on success, -errno on invalid input strings. Error values:
  *
-- 
2.17.1