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Subject[PATCH 6/8] lib: bitmap: support "N" as an alias for size of bitmap
Date
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@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Suggested-by: Yury Norov <yury.norov@gmail.com> # move it from CPU code
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
.../admin-guide/kernel-parameters.rst | 7 ++++++
lib/bitmap.c | 22 ++++++++++++++-----
2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst
index 682ab28b5c94..7733a773f5f8 100644
--- a/Documentation/admin-guide/kernel-parameters.rst
+++ b/Documentation/admin-guide/kernel-parameters.rst
@@ -68,6 +68,13 @@ 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.
+
+Keep in mind that "N" is dynamic, so if system changes cause the bitmap width
+to change, such as less cores in the CPU list, then N and any ranges using N
+will also change. Use the same on a small 4 core system, and "16-N" becomes
+"16-3" and now the same boot input will be flagged as invalid (start > end).


This document may not be entirely up to date and comprehensive. The command
diff --git a/lib/bitmap.c b/lib/bitmap.c
index 833f152a2c43..9f4626a4c95f 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 *num,
+ unsigned int lastbit)
{
unsigned long long n;
unsigned int len;

+ if (str[0] == 'N') {
+ *num = lastbit;
+ return str + 1;
+ }
+
len = _parse_integer(str, 10, &n);
if (!len)
return ERR_PTR(-EINVAL);
@@ -571,7 +577,9 @@ static const char *bitmap_find_region_reverse(const char *start, const char *end

static const char *bitmap_parse_region(const char *str, struct region *r)
{
- str = bitmap_getnum(str, &r->start);
+ unsigned int lastbit = r->nbits - 1;
+
+ str = bitmap_getnum(str, &r->start, lastbit);
if (IS_ERR(str))
return str;

@@ -581,7 +589,7 @@ 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);
+ str = bitmap_getnum(str + 1, &r->end, lastbit);
if (IS_ERR(str))
return str;

@@ -591,14 +599,14 @@ static const char *bitmap_parse_region(const char *str, struct region *r)
if (*str != ':')
return ERR_PTR(-EINVAL);

- str = bitmap_getnum(str + 1, &r->off);
+ str = bitmap_getnum(str + 1, &r->off, lastbit);
if (IS_ERR(str))
return str;

if (*str != '/')
return ERR_PTR(-EINVAL);

- return bitmap_getnum(str + 1, &r->group_len);
+ return bitmap_getnum(str + 1, &r->group_len, lastbit);

no_end:
r->end = r->start;
@@ -625,6 +633,10 @@ 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). Keep in mind that it is
+ * dynamic, so if system changes cause the bitmap width to change, such
+ * as more cores in a CPU list, then any ranges using N will also change.
*
* Returns: 0 on success, -errno on invalid input strings. Error values:
*
--
2.30.0
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