[PATCH v3 0/7] sysctl: Remove sentinel elements from arch

Joel Granados via B4 Relay devnull+j.granados.samsung.com at kernel.org
Mon Oct 2 22:30:35 AEDT 2023

From: Joel Granados <j.granados at samsung.com>

These commits remove the sentinel element (last empty element) from the
sysctl arrays of all the files under the "arch/" directory that use a
sysctl array for registration. The merging of the preparation patches
(in https://lore.kernel.org/all/ZO5Yx5JFogGi%2FcBo@bombadil.infradead.org/)
to mainline allows us to just remove sentinel elements without changing
behavior. This is now safe because the sysctl registration code
(register_sysctl() and friends) use the array size in addition to
checking for a sentinel ([1] for more info).

These commits are part of a bigger set (bigger patchset here
that remove the ctl_table sentinel. The idea is to make the review
process easier by chunking the 52 commits into manageable pieces. By
sending out one chunk at a time, they can be reviewed separately without
noise from parallel sets. After the "arch/" commits in this set are
reviewed, I will continue with drivers/*, fs/*, kernel/*, net/* and
miscellaneous. The final set will remove the unneeded check for
->procname == NULL.

By removing the sysctl sentinel elements we avoid kernel bloat as
ctl_table arrays get moved out of kernel/sysctl.c into their own
respective subsystems. This move was started long ago to avoid merge
conflicts; the sentinel removal bit came after Mathew Wilcox suggested
it to avoid bloating the kernel by one element as arrays moved out. This
patchset will reduce the overall build time size of the kernel and run
time memory bloat by about ~64 bytes per declared ctl_table array. I
have consolidated some links that shed light on the history of this
effort [2].

* Added clarification both in the commit messages and the coverletter as
  to why this patch is safe to apply.
* Added {Acked,Reviewed,Tested}-by from list
* Link to v1: https://lore.kernel.org/r/20230906-jag-sysctl_remove_empty_elem_arch-v1-0-3935d4854248@samsung.com

* Removed the ia64 patch to avoid conflicts with the ia64 removal
* Rebased onto v6.6-rc4
* Kept/added the trailing comma for the ctl_table arrays. This was a comment
  that we received "drivers/*" patch set.
* Link to v2: https://lore.kernel.org/r/20230913-jag-sysctl_remove_empty_elem_arch-v2-0-d1bd13a29bae@samsung.com

* Ran sysctl selftests (./tools/testing/selftests/sysctl/sysctl.sh)
* Ran this through 0-day with no errors or warnings

Size saving after removing all sentinels:
  A consequence of eventually removing all the sentinels (64 bytes per
  sentinel) is the bytes we save. These are *not* numbers that we will
  get after this patch set; these are the numbers that we will get after
  removing all the sentinels. I included them here because they are
  relevant and to get an idea of just how much memory we are talking
    * bloat-o-meter:
        - The "yesall" configuration results save 9158 bytes (bloat-o-meter output here
        - The "tiny" config + CONFIG_SYSCTL save 1215 bytes (bloat-o-meter output here
    * memory usage:
        we save some bytes in main memory as well. In my testing kernel
        I measured a difference of 7296 bytes. I include the way to
        measure in [3]

Size saving after this patchset:
  Here I give the values that I measured for the architecture that I'm
  running (x86_64). This can give an approximation of how many bytes are
  saved for each arch. I won't publish for all the archs because I don't
  have access to all of them.
    * bloat-o-meter
        - The "yesall" config saves 192 bytes (bloat-o-meter output [4])
        - The "tiny" config saves 64 bytes (bloat-o-meter output [5])
    * memory usage:
        In this case there were no bytes saved. To measure it comment the
        printk in `new_dir` and uncomment the if conditional in `new_links`

Comments/feedback greatly appreciated



Links Related to the ctl_table sentinel removal:
* Good summary from Luis sent with the "pull request" for the
  preparation patches.
* Another very good summary from Luis.
* This is a patch set that replaces register_sysctl_table with register_sysctl
* Patch set to deprecate register_sysctl_paths()
* Here there is an explicit expectation for the removal of the sentinel element.
* The "ARRAY_SIZE" approach was mentioned (proposed?) in this thread

To measure the in memory savings apply this on top of this patchset.

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index c88854df0b62..e0073a627bac 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -976,6 +976,8 @@ static struct ctl_dir *new_dir(struct ctl_table_set *set,
        table[0].procname = new_name;
        table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
        init_header(&new->header, set->dir.header.root, set, node, table, 1);
+       // Counts additional sentinel used for each new dir.
+       printk("%ld sysctl saved mem kzalloc \n", sizeof(struct ctl_table));

        return new;
@@ -1199,6 +1201,9 @@ static struct ctl_table_header *new_links(struct ctl_dir *dir, struct ctl_table_
                link_name += len;
+       // Counts additional sentinel used for each new registration
+       //if ((head->ctl_table + head->ctl_table_size)->procname)
+               printk("%ld sysctl saved mem kzalloc \n", sizeof(struct ctl_table));
        init_header(links, dir->header.root, dir->header.set, node, link_table,
        links->nreg = nr_entries;
and then run the following bash script in the kernel:

for n in $(dmesg | grep kzalloc | awk '{print $3}') ; do
    echo $n
    accum=$(calc "$accum + $n")
echo $accum

add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-192 (-192)
Function                                     old     new   delta
sld_sysctls                                  128      64     -64
itmt_kern_table                              128      64     -64
abi_table2                                   128      64     -64
Total: Before=429173594, After=429173402, chg -0.00%

add/remove: 0/0 grow/shrink: 1/0 up/down: 64/0 (64)
Function                                     old     new   delta
sld_sysctls                                   64     128     +64
Total: Before=1886119, After=1886183, chg +0.00%

Signed-off-by: Joel Granados <j.granados at samsung.com>


Joel Granados (7):
      S390: Remove now superfluous sentinel elem from ctl_table arrays
      arm: Remove now superfluous sentinel elem from ctl_table arrays
      arch/x86: Remove now superfluous sentinel elem from ctl_table arrays
      x86/vdso: Remove now superfluous sentinel element from ctl_table array
      riscv: Remove now superfluous sentinel element from ctl_table array
      powerpc: Remove now superfluous sentinel element from ctl_table arrays
      c-sky: Remove now superfluous sentinel element from ctl_talbe array

 arch/arm/kernel/isa.c                     | 4 ++--
 arch/arm64/kernel/armv8_deprecated.c      | 8 +++-----
 arch/arm64/kernel/fpsimd.c                | 2 --
 arch/arm64/kernel/process.c               | 1 -
 arch/csky/abiv1/alignment.c               | 1 -
 arch/powerpc/kernel/idle.c                | 1 -
 arch/powerpc/platforms/pseries/mobility.c | 1 -
 arch/riscv/kernel/vector.c                | 1 -
 arch/s390/appldata/appldata_base.c        | 4 +---
 arch/s390/kernel/debug.c                  | 1 -
 arch/s390/kernel/topology.c               | 1 -
 arch/s390/mm/cmm.c                        | 1 -
 arch/s390/mm/pgalloc.c                    | 1 -
 arch/x86/entry/vdso/vdso32-setup.c        | 1 -
 arch/x86/kernel/cpu/intel.c               | 1 -
 arch/x86/kernel/itmt.c                    | 1 -
 drivers/perf/arm_pmuv3.c                  | 1 -
 17 files changed, 6 insertions(+), 25 deletions(-)
base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
change-id: 20230904-jag-sysctl_remove_empty_elem_arch-81db0a6e6cc4

Best regards,
Joel Granados <j.granados at samsung.com>

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