The following commit has been merged into the core/rcu branch of tip:
Commit-ID: 2c4885d24e64941702a8f81c8e83289823ba35d0
Gitweb: https://git.kernel.org/tip/2c4885d24e64941702a8f81c8e83289823ba35d0
Author: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
AuthorDate: Sun, 21 Feb 2021 03:08:25 -05:00
Committer: Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 08 Mar 2021 14:16:58 -08:00
lib: bitmap: support "N" as an alias for size of bitmap
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> # move it from CPU code
Acked-by: Yury Norov <yury.norov@xxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
Documentation/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 1132796..d6e3f67 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 833f152..9f4626a 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 @@ no_pattern:
* 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:
*
|