[PATCH v5] lib/bitmap.c: enhance bitmap syntax

[Noam Camus]

From: Noam Camus <noamca@xxxxxxxxxxxx>

Today there are platforms with many CPUs (up to 4K).
Trying to boot only part of the CPUs may result in too long string.

For example lets take NPS platform that is part of arch/arc.
This platform have SMP system with 256 cores each with
16 HW threads (SMT machine) where HW thread appears as CPU to the kernel.
In this example there is total of 4K CPUs.
When one tries to boot only part of the HW threads from each core the
string representing the map may be long...
For example if for sake of performance we decided to boot only first half
of HW threads of each core the map will look like:
0-7,16-23,32-39,...,4080-4087

This patch introduce new syntax to accommodate with such use case.
I added an optional postfix to a range of CPUs which will choose
according to given modulo the desired range of reminders i.e.:
    <cpus range>:sed_size/group_size

For example, above map can be described in new syntax like this:
0-4095:8/16

Note that this patch is backward compatible with current syntax.

Signed-off-by: Noam Camus <noamca@xxxxxxxxxxxx>
---
 Documentation/kernel-parameters.txt |   15 ++++++++++
 lib/bitmap.c                        |   50 ++++++++++++++++++++++++++++++++---
 2 files changed, 61 insertions(+), 4 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 623502e..4f1e95b 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -33,6 +33,21 @@ can also be entered as
 Double-quotes can be used to protect spaces in values, e.g.:
 	param="spaces in here"
 
+Some of the kernel parameters takes list of CPUs as value e.g.:
+isolcpus, task_isolation, nohz_full, irqaffinity, rcu_nocbs.
+The format of this list is:
+<cpu number>,...,<cpu number>
+or
+<cpu number>-<cpu number>
+(must be a positive range in ascending order)
+or a mixture
+<cpu number>,...,<cpu number>-<cpu number>
+Note that for special case of range one can split range into equal size of groups
+and for each group to use some amount form the begin of that group.
+<cpu number>-cpu number>:<used size>/<group size>
+For example one can add to its command line following parameter:
+isolcpus=1,2,10-20,100-2000:2/25
+
 This document may not be entirely up to date and comprehensive. The command
 "modinfo -p ${modulename}" shows a current list of all parameters of a loadable
 module. Loadable modules, after being loaded into the running kernel, also
diff --git a/lib/bitmap.c b/lib/bitmap.c
index c66da50..9fb2cbc 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -496,6 +496,11 @@ EXPORT_SYMBOL(bitmap_print_to_pagebuf);
  * ranges.  Consecutively set bits are shown as two hyphen-separated
  * decimal numbers, the smallest and largest bit numbers set in
  * the range.
+ * Optionally each range can be postfixed to denote that only parts of it
+ * should be set. The range will divided to groups of specific size.
+ * 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
  *
  * Returns 0 on success, -errno on invalid input strings.
  * Error values:
@@ -507,16 +512,20 @@ static int __bitmap_parselist(const char *buf, unsigned int buflen,
 		int is_user, unsigned long *maskp,
 		int nmaskbits)
 {
-	unsigned a, b;
+	unsigned int a, b, old_a, old_b;
+	unsigned int group_size, used_size;
 	int c, old_c, totaldigits, ndigits;
 	const char __user __force *ubuf = (const char __user __force *)buf;
-	int at_start, in_range;
+	int at_start, in_range, in_partial_range;
 
 	totaldigits = c = 0;
+	old_a = old_b = 0;
+	group_size = used_size = 0;
 	bitmap_zero(maskp, nmaskbits);
 	do {
 		at_start = 1;
 		in_range = 0;
+		in_partial_range = 0;
 		a = b = 0;
 		ndigits = totaldigits;
 
@@ -547,6 +556,24 @@ static int __bitmap_parselist(const char *buf, unsigned int buflen,
 			if ((totaldigits != ndigits) && isspace(old_c))
 				return -EINVAL;
 
+			if (c == '/') {
+				used_size = a;
+				at_start = 1;
+				in_range = 0;
+				a = b = 0;
+				continue;
+			}
+
+			if (c == ':') {
+				old_a = a;
+				old_b = b;
+				at_start = 1;
+				in_range = 0;
+				in_partial_range = 1;
+				a = b = 0;
+				continue;
+			}
+
 			if (c == '-') {
 				if (at_start || in_range)
 					return -EINVAL;
@@ -567,15 +594,30 @@ static int __bitmap_parselist(const char *buf, unsigned int buflen,
 		}
 		if (ndigits == totaldigits)
 			continue;
+		if (in_partial_range) {
+			group_size = a;
+			a = old_a;
+			b = old_b;
+			old_a = old_b = 0;
+		}
 		/* if no digit is after '-', it's wrong*/
 		if (at_start && in_range)
 			return -EINVAL;
-		if (!(a <= b))
+		if (!(a <= b) || !(used_size <= group_size))
 			return -EINVAL;
 		if (b >= nmaskbits)
 			return -ERANGE;
 		while (a <= b) {
-			set_bit(a, maskp);
+			if (in_partial_range) {
+				static int pos_in_group = 1;
+
+				if (pos_in_group <= used_size)
+					set_bit(a, maskp);
+
+				if (a == b || ++pos_in_group > group_size)
+					pos_in_group = 1;
+			} else
+				set_bit(a, maskp);
 			a++;
 		}
 	} while (buflen && c == ',');
-- 
1.7.1