[PATCH v6 0/2] rcu/nocb: Delete local variable 'need_rcu_nocb_mask' in rcu_init_nohz()

[Zhen Lei]

v5 --> v6:
1. Frist, update patch 1/2, so that, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y takes
   effect only when there are no 'rcu_nocbs=' and 'nohz_full='. Retain the same
   semantics as the original.
2. Then, add patch 2/2, so that, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y takes effect
   as long as there is no "rcu_nocbs=" boot parameter. This is suggested by
   Frederic Weisbecker.

Test cases and test results as below:
 ----------------------------------------------------------------------
| CONFIG_NO_HZ_FULL  | CONFIG_RCU_NOCB_CPU_DEFAULT_ALL |    cmdline    |
 ----------------------------------------------------------------------
1)         N         |                 N               |     none      |
2)         N         |                 N               | rcu_nocbs=1-2 |
3)         Y         |                 N               | nohz_full=2-3 |
4)         Y         |                 N               | rcu_nocbs=1 nohz_full=3 |
5)         Y         |                 Y               |      none     |
6)         Y         |                 Y               | rcu_nocbs=1-2 |
7)         Y         |                 Y               | nohz_full=2-3 |
8)         Y         |                 Y               | rcu_nocbs=1 nohz_full=3 |
 ----------------------------------------------------------------------

dmesg | grep "Offload RCU callbacks"
1) none
2) Offload RCU callbacks from CPUs: 1-2.
3) Offload RCU callbacks from CPUs: 2-3.
4) Offload RCU callbacks from CPUs: 1,3.
5) Offload RCU callbacks from CPUs: 0-3.
6) Offload RCU callbacks from CPUs: 1-2.
7) Offload RCU callbacks from CPUs: 0-3.
8) Offload RCU callbacks from CPUs: 1,3.


v4 --> v5:
1. Cancel eliminate rcu_state.nocb_is_setup. It is useful for case 1) below.

Test cases and test results as below:
 ----------------------------------------------------------------------
| CONFIG_NO_HZ_FULL  | CONFIG_RCU_NOCB_CPU_DEFAULT_ALL |    cmdline    |
 ----------------------------------------------------------------------
1)         N         |                 N               |     none      |
2)         N         |                 N               | rcu_nocbs=1-2 |
3)         Y         |                 N               | nohz_full=1-2 |
4)         Y         |                 Y               |     none      |
 ----------------------------------------------------------------------
 
1)
[ 1018.082417] rcu-torture:--- End of test: SUCCESS (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"

root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
# CONFIG_NO_HZ_FULL is not set
root@genericarmv8:~# cat /proc/cmdline
console=ttyS0 earlyprintk=serial root=/dev/sda rw

2) 3) 4)
The same to v4.


v3 --> v4:
Fix a build warning.
-       struct cpumask *cpumask = NULL;
+       const struct cpumask *cpumask = NULL;

v2 --> v3:
1. Eliminate rcu_state.nocb_is_setup
2. Update the code based on the above patch and commit b37a667c6242
   ("rcu/nocb: Add an option to offload all CPUs on boot").

Test cases and test results as below:

 ----------------------------------------------------------------------
| CONFIG_NO_HZ_FULL  | CONFIG_RCU_NOCB_CPU_DEFAULT_ALL |    cmdline    |
 ----------------------------------------------------------------------
1)         N         |                 N               |     none      |
2)         N         |                 N               | rcu_nocbs=1-2 |
3)         Y         |                 N               | nohz_full=1-2 |
4)         Y         |                 Y               |     none      |
 ----------------------------------------------------------------------

1)
[ 1085.691770] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: (none).
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
# CONFIG_NO_HZ_FULL is not set
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000

2)
[ 1022.532198] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: 1-2.
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
# CONFIG_NO_HZ_FULL is not set
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000 rcu_nocbs=1-2

3)
[ 1011.714712] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: 1-2.
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
CONFIG_NO_HZ_FULL=y
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000 nohz_full=1-2

4)
[ 1031.047094] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: 0-3.
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
CONFIG_NO_HZ_FULL=y
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000


v1 --> v2:
Update commit message.

Zhen Lei (2):
  rcu: Simplify rcu_init_nohz() cpumask handling
  rcu: Offload callback processing from all CPUs in the absence of
    rcu_nocbs=

 kernel/rcu/Kconfig     |  4 ++--
 kernel/rcu/tree_nocb.h | 35 ++++++++++++-----------------------
 2 files changed, 14 insertions(+), 25 deletions(-)

-- 
2.25.1