[PATCH v2] Documentation/no_hz: Remove description that states boot CPU cannot be nohz_full

[Krishanth Jagaduri via B4 Relay]

From: Oleg Nesterov <oleg@xxxxxxxxxx>

sched/isolation: Prevent boot crash when the boot CPU is nohz_full

[ 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").

However after:

  aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work")

the kernel will crash at boot time in this case; housekeeping_any_cpu()
returns an invalid CPU number until smp_init() brings the first
housekeeping CPU up.

Change housekeeping_any_cpu() to check the result of cpumask_any_and() and
return smp_processor_id() in this case.

This is just the simple and backportable workaround which fixes the
symptom, but smp_processor_id() at boot time should be safe at least for
type == HK_TYPE_TIMER, this more or less matches the tick_do_timer_boot_cpu
logic.

There is no worry about cpu_down(); tick_nohz_cpu_down() will not allow to
offline tick_do_timer_cpu (the 1st online housekeeping CPU).

[ Apply only documentation changes as commit which causes boot
  crash when boot CPU is nohz_full is not backported to stable
  kernels - Krishanth ]

Fixes: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work")
Reported-by: Chris von Recklinghausen <crecklin@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Phil Auld <pauld@xxxxxxxxxx>
Acked-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240411143905.GA19288@xxxxxxxxxx
Closes: https://lore.kernel.org/all/20240402105847.GA24832@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").

When trying LTS kernels between 5.4 and 6.6, we noticed we could use
boot CPU as nohz_full but the information in the document was misleading. 

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 upstream commit as a whole will be backported to
stable kernels which does not contain the commit that introduced the
issue of boot crash when boot CPU is nohz_full.

Could we fix only the document portion in stable kernels 5.4+ that
mentions boot CPU cannot be nohz_full?
---
Changes in v2:
- Add original changelog and trailers to commit message.
- Add backport note for why only document portion is modified.
- Link to v1: https://lore.kernel.org/r/20250205-send-oss-20250129-v1-1-d404921e6d7e@xxxxxxxx
---
 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>