cgroup: improve conversion from shares to weight

The OCI CPU shares (range [2-262144]) to cgroup v2
`cpu.weight` (range [1-10000]) conversion formula has been
updated to use a quadratic function so that min, max and default
values match.

Closes: #1721

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Author: giuseppe

configure.ac

@@ -321,6 +321,8 @@ fi
 
 AC_SEARCH_LIBS([argp_parse], [argp], [], [AC_MSG_ERROR([*** argp functions not found - install libargp or argp_standalone])])
 
+AC_SEARCH_LIBS([log2], [m], [], [AC_MSG_ERROR([*** -lm not found])])
+
 AM_CONDITIONAL([PYTHON_BINDINGS], [test "x$with_python_bindings" = "xyes"])
 AM_CONDITIONAL([LUA_BINDINGS], [test "x$with_lua_bindings" = "xyes"])
 AM_CONDITIONAL([CRIU_SUPPORT], [test "x$have_criu" = "xyes"])

crun.1

@@ -836,7 +836,10 @@ allbox;
 l l l l 
 l l l l .
 \fBOCI (x)\fP	\fBcgroup 2 value (y)\fP	\fBconversion\fP	\fBcomment\fP
-shares	cpu.weight	y = (1 + ((x - 2) * 9999) / 262142)	T{
+shares	cpu.weight	T{
+y=10^((log2(x)^2 + 125 * log2(x)) / 612.0 - 7.0 / 34.0)
+T}
+	T{
 convert from [2-262144] to [1-10000]
 T}
 period	cpu.max	y = x	T{

crun.1.md

@@ -702,11 +702,11 @@ they are converted when needed from the cgroup v1 configuration.
 
 ## CPU controller
 
-| OCI (x) | cgroup 2 value (y) | conversion  |  comment |
-|---|---|---|---|
-| shares | cpu.weight | y = (1 + ((x - 2) \* 9999) / 262142) | convert from [2-262144] to [1-10000]|
-| period | cpu.max | y = x| period and quota are written together|
-| quota | cpu.max | y = x| period and quota are written together|
+| OCI (x) | cgroup 2 value (y) | conversion                                              | comment                               |
+|---------|--------------------|---------------------------------------------------------|---------------------------------------|
+| shares  | cpu.weight         | y=10^((log2(x)^2 + 125 * log2(x)) / 612.0 - 7.0 / 34.0) | convert from [2-262144] to [1-10000]  |
+| period  | cpu.max            | y = x                                                   | period and quota are written together |
+| quota   | cpu.max            | y = x                                                   | period and quota are written together |
 
 ## blkio controller
 

src/libcrun/cgroup-internal.h

@@ -21,6 +21,9 @@
 #include "container.h"
 #include "utils.h"
 
+#include <stdint.h>
+#include <math.h>
+
 enum
 {
   CGROUP_MEMORY = 1 << 0,
@@ -76,8 +79,26 @@ int libcrun_cgroup_pause_unpause_path (const char *cgroup_path, const bool pause
 static inline uint64_t
 convert_shares_to_weight (uint64_t shares)
 {
-  /* convert linearly from 2-262144 to 1-10000.  */
-  return (1 + ((shares - 2) * 9999) / 262142);
+  /* Converts OCI shares (2-262144) to cgroup v2 cpu.weight (1-10000).
+     This uses the same formula as systemd, differing from the earlier linear conversion.
+     The result is clamped to ensure it falls within the valid weight range.  */
+
+  double l, exponent;
+
+  /* The value of 0 means "unset".  */
+  if (shares == 0)
+    return 0;
+  if (shares < 2)
+    return 1;
+  if (shares > 262144)
+    return 10000;
+
+  l = log2 ((double) shares);
+
+  /* Quadratic function which fits min, max, and default.  */
+  exponent = (l * l + 125 * l) / 612.0 - 7.0 / 34.0;
+
+  return (uint64_t) ceil (pow (10, exponent));
 }
 
 int initialize_cpuset_subsystem (const char *path, libcrun_error_t *err);

tests/test_resources.py

@@ -291,13 +291,14 @@ def test_resources_cpu_weight_systemd():
             sys.stderr.write("found wrong CPUWeight for the systemd scope\n")
             return 1
 
-        run_crun_command(['update', '--cpu-share', '4321', cid])
+        cpu_shares = 4321
+        run_crun_command(['update', '--cpu-share', str(cpu_shares), cid])
         # this is the expected cpu weight after the conversion from the CPUShares
-        expected_weight = "165"
+        expected_weight = str(max(1, min(10000, cpu_shares * 100 // 1024)))
 
         out = run_crun_command(["exec", cid, "/init", "cat", "/sys/fs/cgroup/cpu.weight"])
         if expected_weight not in out:
-            sys.stderr.write("found wrong CPUWeight %s for the container cgroup\n" % out)
+            sys.stderr.write("found wrong CPUWeight %s instead of %s for the container cgroup\n" % (out, expected_weight))
             return -1
 
         out = subprocess.check_output(['systemctl', 'show','-PCPUWeight', scope ], close_fds=False).decode().strip()