[PATCH] Documentation/no_hz: Remove description that states boot CPU cannot be nohz_full

[PATCH] Documentation/no_hz: Remove description that states boot CPU cannot be nohz_full

[Krishanth Jagaduri via B4 Relay]

From: Oleg Nesterov <oleg@redhat.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@redhat.com>
Cc: stable@vger.kernel.org # 5.4+
Signed-off-by: Krishanth Jagaduri <Krishanth.Jagaduri@sony.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@sony.com>

Re: [PATCH] Documentation/no_hz: Remove description that states boot CPU cannot be nohz_full

[Greg KH]

On Wed, Feb 05, 2025 at 08:32:14AM +0530, Krishanth Jagaduri via B4 Relay wrote:
> From: Oleg Nesterov <oleg@redhat.com>
> 
> [ Upstream commit 5097cbcb38e6e0d2627c9dde1985e91d2c9f880e ]

It's just the documentation part of that commit, not the full one.

> 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.

You dropped the rest of the changelog text here :(

> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Cc: stable@vger.kernel.org # 5.4+
> Signed-off-by: Krishanth Jagaduri <Krishanth.Jagaduri@sony.com>

And you dropped all the other signed-off-by lines :(

> ---
> 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+?

Does the documentation lines really matter here?

At the very least, we can't take this as the signed-off-by lines are all
gone.  Please resend with them all back, and then make a note that you
are only including the documentation portion and why.

thanks,

greg k-h

Re: [PATCH] Documentation/no_hz: Remove description that states boot CPU cannot be nohz_full

[Krishanth Jagaduri]

On Wed, Feb 05, 2025 at 10:18:17AM +0100, Greg KH wrote:
> On Wed, Feb 05, 2025 at 08:32:14AM +0530, Krishanth Jagaduri via B4 Relay wrote:
> > From: Oleg Nesterov <oleg@redhat.com>
> > 
> > [ Upstream commit 5097cbcb38e6e0d2627c9dde1985e91d2c9f880e ]
> 
> It's just the documentation part of that commit, not the full one.
> 

Updated in v2. Thank you.

> > 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.
> 
> You dropped the rest of the changelog text here :(
>

Updated in v2. Thank you.

> > 
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > Cc: stable@vger.kernel.org # 5.4+
> > Signed-off-by: Krishanth Jagaduri <Krishanth.Jagaduri@sony.com>
> 
> And you dropped all the other signed-off-by lines :(
>

Updated in v2. Thank you.

> > 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+?
> 
> Does the documentation lines really matter here?
> 

When we tried LTS kernels 5.4 to 6.6, we noticed that boot CPU can be
nohz_full without any problems. But information in documentation was
misleading. We wanted to check if it would be okay to fix the information
in the document.

> At the very least, we can't take this as the signed-off-by lines are all
> gone.  Please resend with them all back, and then make a note that you
> are only including the documentation portion and why.
>
Updated in v2. Thank you.

Best regards,
Krishanth