[patch 14/19] softirq: Make softirq control and processing RT aware

Thomas Gleixner
2020-11-13T14:24:58Z

Provide a local lock based serialization for soft interrupts on RT which
allows the local_bh_disabled() sections and servicing soft interrupts to be
preemptible.

Provide the necessary inline helpers which allow to reuse the bulk of the
softirq processing code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
include/linux/bottom_half.h | 2
kernel/softirq.c | 207 ++++++++++++++++++++++++++++++++++++++++++--
2 files changed, 201 insertions(+), 8 deletions(-)

--- a/include/linux/bottom_half.h
+++ b/include/linux/bottom_half.h
@@ -4,7 +4,7 @@

#include <linux/preempt.h>

-#ifdef CONFIG_TRACE_IRQFLAGS
+#if defined(CONFIG_PREEMPT_RT) || defined(CONFIG_TRACE_IRQFLAGS)
extern void __local_bh_disable_ip(unsigned long ip, unsigned int cnt);
#else
static __always_inline void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -13,6 +13,7 @@
#include <linux/kernel_stat.h>
#include <linux/interrupt.h>
#include <linux/init.h>
+#include <linux/local_lock.h>
#include <linux/mm.h>
#include <linux/notifier.h>
#include <linux/percpu.h>
@@ -100,20 +101,208 @@ EXPORT_PER_CPU_SYMBOL_GPL(hardirq_contex
#endif

/*
- * preempt_count and SOFTIRQ_OFFSET usage:
- * - preempt_count is changed by SOFTIRQ_OFFSET on entering or leaving
- * softirq processing.
- * - preempt_count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
+ * SOFTIRQ_OFFSET usage:
+ *
+ * On !RT kernels 'count' is the preempt counter, on RT kernels this applies
+ * to a per CPU counter and to task::softirqs_disabled_cnt.
+ *
+ * - count is changed by SOFTIRQ_OFFSET on entering or leaving softirq
+ * processing.
+ *
+ * - count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
* on local_bh_disable or local_bh_enable.
+ *
* This lets us distinguish between whether we are currently processing
* softirq and whether we just have bh disabled.
*/
+#ifdef CONFIG_PREEMPT_RT

-#ifdef CONFIG_TRACE_IRQFLAGS
/*
- * This is for softirq.c-internal use, where hardirqs are disabled
+ * RT accounts for BH disabled sections in task::softirqs_disabled_cnt and
+ * also in per CPU softirq_ctrl::cnt. This is necessary to allow tasks in a
+ * softirq disabled section to be preempted.
+ *
+ * The per task counter is used for softirq_count(), in_softirq() and
+ * in_serving_softirqs() because these counts are only valid when the task
+ * holding softirq_ctrl::lock is running.
+ *
+ * The per CPU counter prevents pointless wakeups of ksoftirqd in case that
+ * the task which is in a softirq disabled section is preempted or blocks.
+ */
+struct softirq_ctrl {
+ local_lock_t lock;
+ int cnt;
+};
+
+static DEFINE_PER_CPU(struct softirq_ctrl, softirq_ctrl) = {
+ .lock = INIT_LOCAL_LOCK(softirq_ctrl.lock),
+};
+
+void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
+{
+ unsigned long flags;
+ int newcnt;
+
+ WARN_ON_ONCE(in_hardirq());
+
+ /* First entry of a task into a BH disabled section? */
+ if (!current->softirq_disable_cnt) {
+ if (preemptible()) {
+ local_lock(&softirq_ctrl.lock);
+ rcu_read_lock();
+ } else {
+ DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt));
+ }
+ }
+
+ preempt_disable();
+ /*
+ * 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_TRACE_IRQFLAGS) && newcnt == cnt) {
+ raw_local_irq_save(flags);
+ lockdep_softirqs_off(ip);
+ raw_local_irq_restore(flags);
+ }
+ preempt_enable();
+}
+EXPORT_SYMBOL(__local_bh_disable_ip);
+
+static void __local_bh_enable(unsigned int cnt, bool unlock)
+{
+ unsigned long flags;
+ int newcnt;
+
+ DEBUG_LOCKS_WARN_ON(current->softirq_disable_cnt !=
+ this_cpu_read(softirq_ctrl.cnt));
+
+ preempt_disable();
+ if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && softirq_count() == cnt) {
+ 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;
+ preempt_enable();
+
+ if (!newcnt && unlock) {
+ rcu_read_unlock();
+ local_unlock(&softirq_ctrl.lock);
+ }
+}
+
+void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
+{
+ bool preempt_on = preemptible();
+ unsigned long flags;
+ u32 pending;
+ int curcnt;
+
+ WARN_ON_ONCE(in_irq());
+ lockdep_assert_irqs_enabled();
+
+ local_irq_save(flags);
+ curcnt = this_cpu_read(softirq_ctrl.cnt);
+
+ /*
+ * If this is not reenabling soft interrupts, no point in trying to
+ * run pending ones.
+ */
+ if (curcnt != cnt)
+ goto out;
+
+ pending = local_softirq_pending();
+ if (!pending || ksoftirqd_running(pending))
+ goto out;
+
+ /*
+ * If this was called from non preemptible context, wake up the
+ * softirq daemon.
+ */
+ if (!preempt_on) {
+ wakeup_softirqd();
+ goto out;
+ }
+
+ /*
+ * Adjust softirq count to SOFTIRQ_OFFSET which makes
+ * in_serving_softirq() become true.
+ */
+ cnt = SOFTIRQ_OFFSET;
+ __local_bh_enable(cnt, false);
+ __do_softirq();
+
+out:
+ __local_bh_enable(cnt, preempt_on);
+ local_irq_restore(flags);
+}
+EXPORT_SYMBOL(__local_bh_enable_ip);
+
+/*
+ * Invoked from irq_enter_rcu() to prevent that tick_irq_enter()
+ * pointlessly wakes the softirq daemon. That's handled in __irq_exit_rcu().
+ * None of the above logic in the regular bh_disable/enable functions is
+ * required here.
+ */
+static inline void local_bh_disable_irq_enter(void)
+{
+ this_cpu_add(softirq_ctrl.cnt, SOFTIRQ_DISABLE_OFFSET);
+}
+
+static inline void local_bh_enable_irq_enter(void)
+{
+ this_cpu_sub(softirq_ctrl.cnt, SOFTIRQ_DISABLE_OFFSET);
+}
+
+/*
+ * Invoked from ksoftirqd_run() outside of the interrupt disabled section
+ * to acquire the per CPU local lock for reentrancy protection.
+ */
+static inline void ksoftirqd_run_begin(void)
+{
+ __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
+ local_irq_disable();
+}
+
+/* Counterpart to ksoftirqd_run_begin() */
+static inline void ksoftirqd_run_end(void)
+{
+ __local_bh_enable(SOFTIRQ_OFFSET, true);
+ WARN_ON_ONCE(in_interrupt());
+ local_irq_enable();
+}
+
+static inline void softirq_handle_begin(void) { }
+static inline void softirq_handle_end(void) { }
+
+static inline void invoke_softirq(void)
+{
+ if (!this_cpu_read(softirq_ctrl.cnt))
+ wakeup_softirqd();
+}
+
+static inline bool should_wake_ksoftirqd(void)
+{
+ return !this_cpu_read(softirq_ctrl.cnt);
+}
+
+#else /* CONFIG_PREEMPT_RT */
+
+/*
+ * This one is for softirq.c-internal use, where hardirqs are disabled
* legitimately:
*/
+#ifdef CONFIG_TRACE_IRQFLAGS
void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
{
unsigned long flags;
@@ -284,6 +473,8 @@ asmlinkage __visible void do_softirq(voi
local_irq_restore(flags);
}

+#endif /* !CONFIG_PREEMPT_RT */
+
/*
* We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
* but break the loop if need_resched() is set or after 2 ms.
@@ -388,8 +579,10 @@ asmlinkage __visible void __softirq_entr
pending >>= softirq_bit;
}

- if (__this_cpu_read(ksoftirqd) == current)
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
+ __this_cpu_read(ksoftirqd) == current)
rcu_softirq_qs();
+
local_irq_disable();

pending = local_softirq_pending();