linux-kernel - [PATCH] Documentation/no_hz: Remove description that states boot CPU cannot be nohz

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20250205-send-oss-20250129-v1-1-d404921e6d7e@sony.com>
Date: Wed, 05 Feb 2025 08:32:14 +0530
From: Krishanth Jagaduri via B4 Relay <devnull+Krishanth.Jagaduri.sony.com@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>, Jonathan Corbet <corbet@....net>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
 Atsushi Ochiai <Atsushi.Ochiai@...y.com>, 
 Daniel Palmer <Daniel.Palmer@...y.com>, Oleg Nesterov <oleg@...hat.com>, 
 stable@...r.kernel.org, Krishanth Jagaduri <Krishanth.Jagaduri@...y.com>
Subject: [PATCH] Documentation/no_hz: Remove description that states boot
 CPU cannot be nohz_full

From: Oleg Nesterov <oleg@...hat.com>

[ 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@...hat.com>
Cc: stable@...r.kernel.org # 5.4+
Signed-off-by: Krishanth Jagaduri <Krishanth.Jagaduri@...y.com>
---
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@...y.com>