From: Oleg Nesterov <oleg@xxxxxxxxxx>
[ Upstream commit 5097cbcb38e6e0d2627c9dde1985e91d2c9f880e ]
Documentation/timers/no_hz.rst states that the "nohz_full=" mask must not
include the boot CPU, which is no longer true after:
commit 08ae95f4fd3b ("nohz_full: Allow the boot CPU to be nohz_full").
Apply changes only to Documentation/timers/no_hz.rst in stable kernels.
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 5.4+
Signed-off-by: Krishanth Jagaduri <Krishanth.Jagaduri@xxxxxxxx>
---
Hi,
Before kernel 6.9, Documentation/timers/no_hz.rst states that
"nohz_full=" mask must not include the boot CPU, which is no longer
true after commit 08ae95f4fd3b ("nohz_full: Allow the boot CPU to be
nohz_full").
This was fixed upstream by commit 5097cbcb38e6 ("sched/isolation: Prevent
boot crash when the boot CPU is nohz_full").
While it fixes the document description, it also fixes issue introduced
by another commit aae17ebb53cd ("workqueue: Avoid using isolated cpus'
timers on queue_delayed_work").
It is unlikely that it will be backported to stable kernels which does
not contain the commit that introduced the issue.
Could Documentation/timers/no_hz.rst be fixed in stable kernels 5.4+?
---
Documentation/timers/no_hz.rst | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
index 065db217cb04fc252bbf6a05991296e7f1d3a4c5..16bda468423e88090c0dc467ca7a5c7f3fd2bf02 100644
--- a/Documentation/timers/no_hz.rst
+++ b/Documentation/timers/no_hz.rst
@@ -129,11 +129,8 @@ adaptive-tick CPUs: At least one non-adaptive-tick CPU must remain
online to handle timekeeping tasks in order to ensure that system
calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
-user processes to observe slight drifts in clock rate.) Therefore, the
-boot CPU is prohibited from entering adaptive-ticks mode. Specifying a
-"nohz_full=" mask that includes the boot CPU will result in a boot-time
-error message, and the boot CPU will be removed from the mask. Note that
-this means that your system must have at least two CPUs in order for
+user processes to observe slight drifts in clock rate.) Note that this
+means that your system must have at least two CPUs in order for
CONFIG_NO_HZ_FULL=y to do anything for you.
Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
---
base-commit: 219d54332a09e8d8741c1e1982f5eae56099de85
change-id: 20250129-send-oss-20250129-3c42dcf463eb
Best regards,
--
Krishanth Jagaduri <Krishanth.Jagaduri@xxxxxxxx>
