[PATCH 0/8] sysctl: Remove sentinel elements from arch

Joel Granados via B4 Relay
2023-09-06T10:04:13Z

What?
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 (more info on how this was done here [1]).

These commits are part of a bigger set (bigger patchset here
https://github.com/Joelgranados/linux/tree/tag/sysctl_remove_empty_elem_V4)
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.

Why?
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].

Testing:
* 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
about.
* bloat-o-meter:
- The "yesall" configuration results save 9158 bytes (bloat-o-meter output here
https://lore.kernel.org/all/20230621091000.424843-1-j.granados@samsung.com/)
- The "tiny" config + CONFIG_SYSCTL save 1215 bytes (bloat-o-meter output here
https://lore.kernel.org/all/20230809105006.1198165-1-j.granados@samsung.com/)
* 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`
[3].

Comments/feedback greatly appreciated

Best
Joel

[1]
We are able to remove a sentinel table without behavioral change by
introducing a table_size argument in the same place where procname is
checked for NULL. The idea is for it to keep stopping when it hits
->procname == NULL, while the sentinel is still present. And when the
sentinel is removed, it will stop on the table_size. You can go to
(https://lore.kernel.org/all/20230809105006.1198165-1-j.granados@samsung.com/)
for more information.

[2]
Links Related to the ctl_table sentinel removal:
* Good summary from Luis sent with the "pull request" for the
preparation patches.
https://lore.kernel.org/all/ZO5Yx5JFogGi%2FcBo@bombadil.infradead.org/
* Another very good summary from Luis.
https://lore.kernel.org/all/ZMFizKFkVxUFtSqa@bombadil.infradead.org/
* This is a patch set that replaces register_sysctl_table with register_sysctl
https://lore.kernel.org/all/20230302204612.782387-1-mcgrof@kernel.org/
* Patch set to deprecate register_sysctl_paths()
https://lore.kernel.org/all/20230302202826.776286-1-mcgrof@kernel.org/
* Here there is an explicit expectation for the removal of the sentinel element.
https://lore.kernel.org/all/20230321130908.6972-1-frank.li@vivo.com
* The "ARRAY_SIZE" approach was mentioned (proposed?) in this thread
https://lore.kernel.org/all/20220220060626.15885-1-tangmeng@uniontech.com

[3]
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;
link++;
}
+ // 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,
head->ctl_table_size);
links->nreg = nr_entries;
"
and then run the following bash script in the kernel:

accum=0
for n in $(dmesg | grep kzalloc | awk '{print $3}') ; do
echo $n
accum=$(calc "$accum + $n")
done
echo $accum

[4]
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%

[5]
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@samsung.com>

---

---
Joel Granados (8):
S390: Remove sentinel elem from ctl_table arrays
arm: Remove sentinel elem from ctl_table arrays
arch/x86: Remove sentinel elem from ctl_table arrays
x86 vdso: rm sentinel element from ctl_table array
riscv: Remove sentinel element from ctl_table array
powerpc: Remove sentinel element from ctl_table arrays
ia64: Remove sentinel element from ctl_table array
c-sky: rm sentinel element from ctl_talbe array

arch/arm/kernel/isa.c | 4 ++--
arch/arm64/kernel/armv8_deprecated.c | 8 +++-----
arch/arm64/kernel/fpsimd.c | 6 ++----
arch/arm64/kernel/process.c | 3 +--
arch/csky/abiv1/alignment.c | 3 +--
arch/ia64/kernel/crash.c | 3 +--
arch/powerpc/kernel/idle.c | 3 +--
arch/powerpc/platforms/pseries/mobility.c | 3 +--
arch/riscv/kernel/vector.c | 3 +--
arch/s390/appldata/appldata_base.c | 6 ++----
arch/s390/kernel/debug.c | 3 +--
arch/s390/kernel/topology.c | 3 +--
arch/s390/mm/cmm.c | 3 +--
arch/s390/mm/pgalloc.c | 3 +--
arch/x86/entry/vdso/vdso32-setup.c | 3 +--
arch/x86/kernel/cpu/intel.c | 3 +--
arch/x86/kernel/itmt.c | 3 +--
drivers/perf/arm_pmuv3.c | 3 +--
18 files changed, 23 insertions(+), 43 deletions(-)
---
base-commit: 708283abf896dd4853e673cc8cba70acaf9bf4ea
change-id: 20230904-jag-sysctl_remove_empty_elem_arch-81db0a6e6cc4

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