[openwrt/openwrt] x86: switch from CONFIG_HZ_PERIODIC to CONFIG_NO_HZ_IDLE

[LEDE Commits]

robimarko pushed a commit to openwrt/openwrt.git, branch main:
https://git.openwrt.org/92ff3dfd8465fcf2e1e6ce95125c76f88e476cac

commit 92ff3dfd8465fcf2e1e6ce95125c76f88e476cac
Author: John Audia <therealgraysky at proton.me>
AuthorDate: Fri Jan 9 08:16:09 2026 -0500

    x86: switch from CONFIG_HZ_PERIODIC to CONFIG_NO_HZ_IDLE
    
    Running with CONFIG_HZ_PERIODIC=y keeps the scheduler tick running
    continuously, which produces higher jitter and lower power efficiency.
    In contrast, CONFIG_NO_HZ_IDLE=y (the upstream default) stops the tick
    only when the CPU is idle, giving lower idle power and normal runtime
    jitter.
    
    An Intel N150-based router/firewall was tested using two kernel builds:
    one with CONFIG_HZ_PERIODIC=y, and one with CONFIG_NO_HZ_IDLE=y. Power
    consumption was measured while the system was essentially idle (no
    meaningful traffic). The CONFIG_NO_HZ_IDLE=y build consistently used
    less power.
    
    Details: The two power-measurement methods were:
    
     1. PkgWatt from turbostat (software)
     2. Wall-power measurement using a Kill-A-Watt (hardware)
    
    The test began by zeroing the Kill-A-Watt and simultaneously running:
    turbostat --quiet --Summary --interval 10 --show Busy%,PkgWatt
    
    The test duration was defined by the time required for the Kill-A-Watt
    to accumulate 0.005 kWh, after which the average wattage was calculated.
    
    Results:
    +----------------------+-----------------+----------------+-----------+
    |  Metric              |   HZ_PERIODIC   |   NO_HZ_IDLE   | Delta %   |
    +----------------------+-----------------+----------------+-----------+
    |  PkgWatt             |  3.59 ± 0.38    |  3.38 ± 0.34   |  -5.9 %   |
    +----------------------+-----------------+----------------+-----------+
    | Avg wattage at wall  |      12.47      |     12.00      |  -3.77 %  |
    +----------------------+-----------------+----------------+-----------+
    
    The mean PkgWatt difference is 210 mW (5.9%) in favor of
    CONFIG_NO_HZ_IDLE=y, with a t-statistic of ~3.17 and p ≈ 0.002.
    
    Wall-power measurements show a 470 mW (3.77%) reduction under
    CONFIG_NO_HZ_IDLE=y.
    
    Signed-off-by: John Audia <therealgraysky at proton.me>
    Link: https://github.com/openwrt/openwrt/pull/21470
    Signed-off-by: Robert Marko <robimarko at gmail.com>
---
 target/linux/x86/config-6.12 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/linux/x86/config-6.12 b/target/linux/x86/config-6.12
index 5fb0d2885b..581e5a54a5 100644
--- a/target/linux/x86/config-6.12
+++ b/target/linux/x86/config-6.12
@@ -184,7 +184,6 @@ CONFIG_HW_RANDOM=y
 CONFIG_HW_RANDOM_GEODE=y
 CONFIG_HW_RANDOM_VIA=y
 # CONFIG_HYPERVISOR_GUEST is not set
-CONFIG_HZ_PERIODIC=y
 CONFIG_I2C=y
 CONFIG_I2C_ALGOBIT=y
 CONFIG_I2C_BOARDINFO=y
@@ -325,6 +324,7 @@ CONFIG_NLS=y
 # CONFIG_NMI_CHECK_CPU is not set
 # CONFIG_NOHIGHMEM is not set
 CONFIG_NO_HZ=y
+CONFIG_NO_HZ_IDLE=y
 CONFIG_NR_CPUS=1
 CONFIG_NR_CPUS_DEFAULT=1
 CONFIG_NR_CPUS_RANGE_BEGIN=1