[PATCH v2 00/10] sysctl: Remove sentinel elements from kernel dir

Joel Granados via B4 Relay
2024-01-04T15:02:45Z

What?
These commits remove the sentinel element (last empty element) from the
sysctl arrays of all the files under the "kernel/" 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 here [1]).

These commits are part of a bigger set (here
https://github.com/Joelgranados/linux/tree/tag/sysctl_remove_empty_elem_V5)
that remove the ctl_table sentinel. Make the review process easier by
chunking the commits into manageable pieces. Each chunk can be reviewed
separately without noise from parallel sets.

Sending the "kernel/*" chunk now that the "drivers/" has been mostly
reviewed [6]. After this and the "fs/*" are reviewed we only miss 2 more
chunks ("net/*" and miscellaneous) to complete the sentinel removal.
Hurray!!!

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 this patchset:
* bloat-o-meter
- The "yesall" config saves 1984 bytes [4]
- The "tiny" config saves 771 bytes [5]
* If you want to know how many bytes are saved after all the chunks
are merged see [3]

Comments/feedback greatly appreciated

Changes in v2:
- No functional changes; I resent it as I did not see it in the latest
sysctl-next. It might be a bit too late to include it in 6.7 version,
but this v2 can be used for 6.8 when it comes out.
- Rebased on top of v6.7-rc6
- Added trailers to the relevant commits.
- Link to v1: https://lore.kernel.org/r/20231107-jag-sysctl_remove_empty_elem_kernel-v1-0-e4ce1388dfa0@samsung.com
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:
* E-mail threads that summarize the sentinel effort
https://lore.kernel.org/all/ZO5Yx5JFogGi%2FcBo@bombadil.infradead.org/
https://lore.kernel.org/all/ZMFizKFkVxUFtSqa@bombadil.infradead.org/
* Replacing the register functions:
https://lore.kernel.org/all/20230302204612.782387-1-mcgrof@kernel.org/
https://lore.kernel.org/all/20230302202826.776286-1-mcgrof@kernel.org/
* E-mail threads discussing prposal
https://lore.kernel.org/all/20230321130908.6972-1-frank.li@vivo.com
https://lore.kernel.org/all/20220220060626.15885-1-tangmeng@uniontech.com

[3]
Size saving after removing all sentinels:
These are the bytes that we save after removing all the sentinels
(this plus all the other chunks). I included them to get an idea of
how much memory we are talking about.
* bloat-o-meter:
- The "yesall" configuration results save 9158 bytes
https://lore.kernel.org/all/20230621091000.424843-1-j.granados@samsung.com/
- The "tiny" config + CONFIG_SYSCTL save 1215 bytes
https://lore.kernel.org/all/20230809105006.1198165-1-j.granados@samsung.com/
* memory usage:
In memory savings are measured to be 7296 bytes. (here is how to
measure [7])

[4]
add/remove: 0/0 grow/shrink: 0/31 up/down: 0/-1984 (-1984)
Function old new delta
watchdog_sysctls 576 512 -64
watchdog_hardlockup_sysctl 128 64 -64
vm_table 1344 1280 -64
uts_kern_table 448 384 -64
usermodehelper_table 192 128 -64
user_table 832 768 -64
user_event_sysctls 128 64 -64
timer_sysctl 128 64 -64
signal_debug_table 128 64 -64
seccomp_sysctl_table 192 128 -64
sched_rt_sysctls 256 192 -64
sched_fair_sysctls 256 192 -64
sched_energy_aware_sysctls 128 64 -64
sched_dl_sysctls 192 128 -64
sched_core_sysctls 384 320 -64
sched_autogroup_sysctls 128 64 -64
printk_sysctls 512 448 -64
pid_ns_ctl_table_vm 128 64 -64
pid_ns_ctl_table 128 64 -64
latencytop_sysctl 128 64 -64
kprobe_sysctls 128 64 -64
kexec_core_sysctls 256 192 -64
kern_table 2560 2496 -64
kern_reboot_table 192 128 -64
kern_panic_table 192 128 -64
kern_exit_table 128 64 -64
kern_delayacct_table 128 64 -64
kern_acct_table 128 64 -64
hung_task_sysctls 448 384 -64
ftrace_sysctls 128 64 -64
bpf_syscall_table 192 128 -64
Total: Before=429912331, After=429910347, chg -0.00%

[5]
add/remove: 0/1 grow/shrink: 0/12 up/down: 0/-771 (-771)
Function old new delta
sched_core_sysctl_init 43 40 -3
vm_table 1024 960 -64
uts_kern_table 448 384 -64
usermodehelper_table 192 128 -64
user_table 576 512 -64
signal_debug_table 128 64 -64
sched_rt_sysctls 256 192 -64
sched_fair_sysctls 128 64 -64
sched_dl_sysctls 192 128 -64
sched_core_sysctls 64 - -64
kern_table 1792 1728 -64
kern_panic_table 128 64 -64
kern_exit_table 128 64 -64
Total: Before=1886645, After=1885874, chg -0.04%

[6]
https://lore.kernel.org/all/20231002-jag-sysctl_remove_empty_elem_drivers-v2-0-02dd0d46f71e@samsung.com

[7]
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
+ //
+ 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

---

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

---
Joel Granados (10):
kernel misc: Remove the now superfluous sentinel elements from ctl_table array
umh: Remove the now superfluous sentinel elements from ctl_table array
ftrace: Remove the now superfluous sentinel elements from ctl_table array
timekeeping: Remove the now superfluous sentinel elements from ctl_table array
seccomp: Remove the now superfluous sentinel elements from ctl_table array
scheduler: Remove the now superfluous sentinel elements from ctl_table array
printk: Remove the now superfluous sentinel elements from ctl_table array
kprobes: Remove the now superfluous sentinel elements from ctl_table array
delayacct: Remove the now superfluous sentinel elements from ctl_table array
bpf: Remove the now superfluous sentinel elements from ctl_table array

kernel/acct.c | 1 -
kernel/bpf/syscall.c | 1 -
kernel/delayacct.c | 1 -
kernel/exit.c | 1 -
kernel/hung_task.c | 1 -
kernel/kexec_core.c | 1 -
kernel/kprobes.c | 1 -
kernel/latencytop.c | 1 -
kernel/panic.c | 1 -
kernel/pid_namespace.c | 1 -
kernel/pid_sysctl.h | 1 -
kernel/printk/sysctl.c | 1 -
kernel/reboot.c | 1 -
kernel/sched/autogroup.c | 1 -
kernel/sched/core.c | 1 -
kernel/sched/deadline.c | 1 -
kernel/sched/fair.c | 1 -
kernel/sched/rt.c | 1 -
kernel/sched/topology.c | 1 -
kernel/seccomp.c | 1 -
kernel/signal.c | 1 -
kernel/stackleak.c | 1 -
kernel/sysctl.c | 2 --
kernel/time/timer.c | 1 -
kernel/trace/ftrace.c | 1 -
kernel/trace/trace_events_user.c | 1 -
kernel/ucount.c | 3 +--
kernel/umh.c | 1 -
kernel/utsname_sysctl.c | 1 -
kernel/watchdog.c | 2 --
30 files changed, 1 insertion(+), 33 deletions(-)
---
base-commit: ceb6a6f023fd3e8b07761ed900352ef574010bcb
change-id: 20231107-jag-sysctl_remove_empty_elem_kernel-7de90cfd0c0a

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