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.no...@gmail.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: "Paul E. McKenney" <paul...@kernel.org>
Cc: Rasmus Villemoes <li...@rasmusvillemoes.dk>
Cc: Andy Shevchenko <andriy.shevche...@linux.intel.com>
Suggested-by: Yury Norov <yury.no...@gmail.com>
Signed-off-by: Paul Gortmaker <paul.gortma...@windriver.com>
---
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
