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Message-ID: <20250613105653.1860729-2-bigeasy@linutronix.de>
Date: Fri, 13 Jun 2025 12:56:53 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org,
linux-rt-devel@...ts.linux.dev
Cc: Clark Williams <clrkwllms@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
Valentin Schneider <vschneid@...hat.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [RFC PATCH 1/1] softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT
softirqs are preemptible on PREEMPT_RT. There is synchronisation between
individual sections which disable bottom halves. This in turn means that
a forced threaded interrupt can not preempt another forced threaded
interrupt. Instead it will PI-boost the other handler and wait for its
completion.
This is required because code within a softirq section is assumed to be
non-preemptible and may expect exclusive access to per-CPU resources
such as variables or pinned timers.
Code with such expectation has been identified and updated to use
local_lock_nested_bh() for locking of the per-CPU resource. This means
the lock can be removed.
Add CONFIG_PREEMPT_RT_NEEDS_BH_LOCK which keeps the old behaviour if
selected. Otherwise the softirq synchronising is lifted. The
softirq_ctrl.cnt accounting remains to let NOHZ code if softirqs are
currently handled.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
kernel/Kconfig.preempt | 13 +++++++
kernel/softirq.c | 83 ++++++++++++++++++++++++++++++++----------
2 files changed, 76 insertions(+), 20 deletions(-)
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 54ea59ff8fbeb..da326800c1c9b 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -103,6 +103,19 @@ config PREEMPT_RT
Select this if you are building a kernel for systems which
require real-time guarantees.
+config PREEMPT_RT_NEEDS_BH_LOCK
+ bool "Enforce softirq synchronisation on PREEMPT_RT"
+ depends on PREEMPT_RT
+ help
+ Enforce synchronisation across the softirqs context. On PREEMPT_RT
+ the softirq is preemptible. This enforces the same per-CPU BLK
+ semantic non-PREEMPT_RT builds have. This should not be needed
+ because per-CPU locks were added to avoid the per-CPU BKL.
+
+ This switch provides the old behaviour for testing reasons. Select
+ this if you suspect an error with preemptible softirq and want test
+ the old synchronized behaviour.
+
config PREEMPT_COUNT
bool
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 513b1945987cc..7332326fdec8c 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -165,7 +165,11 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
/* First entry of a task into a BH disabled section? */
if (!current->softirq_disable_cnt) {
if (preemptible()) {
- local_lock(&softirq_ctrl.lock);
+ if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK))
+ local_lock(&softirq_ctrl.lock);
+ else
+ migrate_disable();
+
/* Required to meet the RCU bottomhalf requirements. */
rcu_read_lock();
} else {
@@ -177,17 +181,34 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
* Track the per CPU softirq disabled state. On RT this is per CPU
* state to allow preemption of bottom half disabled sections.
*/
- newcnt = __this_cpu_add_return(softirq_ctrl.cnt, cnt);
- /*
- * Reflect the result in the task state to prevent recursion on the
- * local lock and to make softirq_count() & al work.
- */
- current->softirq_disable_cnt = newcnt;
+ if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK)) {
+ newcnt = this_cpu_add_return(softirq_ctrl.cnt, cnt);
+ /*
+ * Reflect the result in the task state to prevent recursion on the
+ * local lock and to make softirq_count() & al work.
+ */
+ current->softirq_disable_cnt = newcnt;
- if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && newcnt == cnt) {
- raw_local_irq_save(flags);
- lockdep_softirqs_off(ip);
- raw_local_irq_restore(flags);
+ if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && newcnt == cnt) {
+ raw_local_irq_save(flags);
+ lockdep_softirqs_off(ip);
+ raw_local_irq_restore(flags);
+ }
+ } else {
+ bool sirq_dis = false;
+
+ if (!current->softirq_disable_cnt)
+ sirq_dis = true;
+
+ this_cpu_add(softirq_ctrl.cnt, cnt);
+ current->softirq_disable_cnt += cnt;
+ WARN_ON_ONCE(current->softirq_disable_cnt < 0);
+
+ if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && sirq_dis) {
+ raw_local_irq_save(flags);
+ lockdep_softirqs_off(ip);
+ raw_local_irq_restore(flags);
+ }
}
}
EXPORT_SYMBOL(__local_bh_disable_ip);
@@ -195,23 +216,42 @@ EXPORT_SYMBOL(__local_bh_disable_ip);
static void __local_bh_enable(unsigned int cnt, bool unlock)
{
unsigned long flags;
+ bool sirq_en = false;
int newcnt;
- DEBUG_LOCKS_WARN_ON(current->softirq_disable_cnt !=
- this_cpu_read(softirq_ctrl.cnt));
+ if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK)) {
+ DEBUG_LOCKS_WARN_ON(current->softirq_disable_cnt !=
+ this_cpu_read(softirq_ctrl.cnt));
+ if (softirq_count() == cnt)
+ sirq_en = true;
+ } else {
+ if (current->softirq_disable_cnt == cnt)
+ sirq_en = true;
+ }
- if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && softirq_count() == cnt) {
+ if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && sirq_en) {
raw_local_irq_save(flags);
lockdep_softirqs_on(_RET_IP_);
raw_local_irq_restore(flags);
}
- newcnt = __this_cpu_sub_return(softirq_ctrl.cnt, cnt);
- current->softirq_disable_cnt = newcnt;
+ if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK)) {
+ newcnt = this_cpu_sub_return(softirq_ctrl.cnt, cnt);
+ current->softirq_disable_cnt = newcnt;
- if (!newcnt && unlock) {
- rcu_read_unlock();
- local_unlock(&softirq_ctrl.lock);
+ if (!newcnt && unlock) {
+ rcu_read_unlock();
+ local_unlock(&softirq_ctrl.lock);
+ }
+ } else {
+ current->softirq_disable_cnt -= cnt;
+ this_cpu_sub(softirq_ctrl.cnt, cnt);
+ if (unlock && !current->softirq_disable_cnt) {
+ migrate_enable();
+ rcu_read_unlock();
+ } else {
+ WARN_ON_ONCE(current->softirq_disable_cnt < 0);
+ }
}
}
@@ -228,7 +268,10 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
lock_map_release(&bh_lock_map);
local_irq_save(flags);
- curcnt = __this_cpu_read(softirq_ctrl.cnt);
+ if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK))
+ curcnt = this_cpu_read(softirq_ctrl.cnt);
+ else
+ curcnt = current->softirq_disable_cnt;
/*
* If this is not reenabling soft interrupts, no point in trying to
--
2.49.0
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