[PATCH v2 0/2] rcu: Make rcu_normal_after_boot writable again

[PATCH v2 0/2] rcu: Make rcu_normal_after_boot writable again

[Juri Lelli]

Hi,

v2 of a proposed RCU change to make rcu_normal_after_boot feature usable
on PREEMPT_RT as well, as it turns out it's useful on RT as well on
NO_HZ_FULL configurations.

v2 gained an additional patch to revise rcu update module parameters
permissions (as suggested during v1 review [1]).

Changes wrt v1:

 - Restrict the option to !PREEMPT_RT || NO_HZ_FULL (Paul, Sebastian)
 - Change module params permissions (Paul) (02/02)

Thanks,
Juri

v1 - https://lore.kernel.org/lkml/20210805080123.16320-1-juri.lelli@redhat.com/

Juri Lelli (2):
  rcu: Make rcu_normal_after_boot writable again
  rcu: Make rcu update module parameters world-readable

 kernel/rcu/tree_exp.h | 1 -
 kernel/rcu/update.c   | 8 ++++----
 2 files changed, 4 insertions(+), 5 deletions(-)

-- 
2.31.1

[PATCH v2 1/2] rcu: Make rcu_normal_after_boot writable again

[Juri Lelli]

Certain configurations (e.g., systems that make heavy use of netns)
need to use synchronize_rcu_expedited() to service RCU grace periods
even after boot.

Even though synchronize_rcu_expedited() has been traditionally
considered harmful for RT for the heavy use of IPIs, it is perfectly
usable under certain conditions (e.g. nohz_full).

Make rcupdate.rcu_normal_after_boot= again writeable on RT (if NO_HZ_
FULL is defined), but keep its default value to 1 (enabled) to avoid
regressions. Users who need synchronize_rcu_expedited() will boot with
rcupdate.rcu_normal_after_ boot=0 in the kernel cmdline.

Reflect the change in synchronize_rcu_expedited_wait() by removing the
WARN related to CONFIG_PREEMPT_RT.

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/rcu/tree_exp.h | 1 -
 kernel/rcu/update.c   | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 2796084ef85a..d9e4f8eb9ae2 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -512,7 +512,6 @@ static void synchronize_rcu_expedited_wait(void)
 		j = READ_ONCE(jiffies_till_first_fqs);
 		if (synchronize_rcu_expedited_wait_once(j + HZ))
 			return;
-		WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT));
 	}
 
 	for (;;) {
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index c21b38cc25e9..bd551134e2f4 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -57,7 +57,7 @@
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
 static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
-#ifndef CONFIG_PREEMPT_RT
+#if !defined(CONFIG_PREEMPT_RT) || defined(CONFIG_NO_HZ_FULL)
 module_param(rcu_normal_after_boot, int, 0);
 #endif
 #endif /* #ifndef CONFIG_TINY_RCU */
-- 
2.31.1

[PATCH v2 2/2] rcu: Make rcu update module parameters world-readable

[Juri Lelli]

rcu update module parameters currently don't appear in sysfs and this is
a serviceability issue as it might be needed to access their default
values at runtime.

Fix this issue by changing rcu update module parameters permissions to
world-readable.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/rcu/update.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index bd551134e2f4..94282dc12bab 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -54,11 +54,11 @@
 #define MODULE_PARAM_PREFIX "rcupdate."
 
 #ifndef CONFIG_TINY_RCU
-module_param(rcu_expedited, int, 0);
-module_param(rcu_normal, int, 0);
+module_param(rcu_expedited, int, 0444);
+module_param(rcu_normal, int, 0444);
 static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
 #if !defined(CONFIG_PREEMPT_RT) || defined(CONFIG_NO_HZ_FULL)
-module_param(rcu_normal_after_boot, int, 0);
+module_param(rcu_normal_after_boot, int, 0444);
 #endif
 #endif /* #ifndef CONFIG_TINY_RCU */
 
-- 
2.31.1

Re: [PATCH v2 0/2] rcu: Make rcu_normal_after_boot writable again

[Paul E. McKenney]

On Tue, Aug 10, 2021 at 10:48:14AM +0200, Juri Lelli wrote:
> Hi,
> 
> v2 of a proposed RCU change to make rcu_normal_after_boot feature usable
> on PREEMPT_RT as well, as it turns out it's useful on RT as well on
> NO_HZ_FULL configurations.
> 
> v2 gained an additional patch to revise rcu update module parameters
> permissions (as suggested during v1 review [1]).

Queued and pushed, thank you!

							Thanx, Paul

> Changes wrt v1:
> 
>  - Restrict the option to !PREEMPT_RT || NO_HZ_FULL (Paul, Sebastian)
>  - Change module params permissions (Paul) (02/02)
> 
> Thanks,
> Juri
> 
> v1 - https://lore.kernel.org/lkml/20210805080123.16320-1-juri.lelli@redhat.com/
> 
> Juri Lelli (2):
>   rcu: Make rcu_normal_after_boot writable again
>   rcu: Make rcu update module parameters world-readable
> 
>  kernel/rcu/tree_exp.h | 1 -
>  kernel/rcu/update.c   | 8 ++++----
>  2 files changed, 4 insertions(+), 5 deletions(-)
> 
> -- 
> 2.31.1
>