[RFC][PATCH 5/9] irq_work: Provide irq_work_queue_remote()

Peter Zijlstra
2020-07-22T15:33:29Z

While the traditional irq_work relies on the ability to self-IPI, it
makes sense to provide an unconditional irq_work_queue_remote()
interface.

This can be used to replace the plagued smp_call_function_single_async().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/irq_work.h | 17 ++++--
kernel/irq_work.c | 129 ++++++++++++++++++++++++++++-------------------
kernel/rcu/tree.c | 6 +-
3 files changed, 95 insertions(+), 57 deletions(-)

--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -46,22 +46,29 @@ static inline bool irq_work_is_busy(stru
return atomic_read(&work->node.a_flags) & IRQ_WORK_BUSY;
}

+#ifdef CONFIG_IRQ_WORK
+
bool irq_work_queue(struct irq_work *work);
bool irq_work_queue_on(struct irq_work *work, int cpu);

void irq_work_tick(void);
void irq_work_sync(struct irq_work *work);

-#ifdef CONFIG_IRQ_WORK
#include <asm/irq_work.h>

void irq_work_run(void);
bool irq_work_needs_cpu(void);
-void irq_work_single(void *arg);
-#else
-static inline bool irq_work_needs_cpu(void) { return false; }
+
+#else /* !CONFIG_IRQ_WORK */
+
static inline void irq_work_run(void) { }
-static inline void irq_work_single(void *arg) { }
+static inline bool irq_work_needs_cpu(void) { return false; }
+
+#endif /* CONFIG_IRQ_WORK */
+
+#ifdef CONFIG_SMP
+extern int irq_work_queue_remote(int cpu, struct irq_work *work);
+extern void irq_work_single(void *arg);
#endif

#endif /* _LINUX_IRQ_WORK_H */
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -20,10 +20,7 @@
#include <linux/smp.h>
#include <asm/processor.h>

-#ifdef CONFIG_IRQ_WORK
-
-static DEFINE_PER_CPU(struct llist_head, raised_list);
-static DEFINE_PER_CPU(struct llist_head, lazy_list);
+#if defined(CONFIG_IRQ_WORK) || defined(CONFIG_SMP)

/*
* Claim the entry so that no one else will poke at it.
@@ -43,6 +40,82 @@ static bool irq_work_claim(struct irq_wo
return true;
}

+void irq_work_single(void *arg)
+{
+ struct irq_work *work = arg;
+ int flags;
+
+ /*
+ * Clear the PENDING bit, after this point the @work can be re-used.
+ * The PENDING bit acts as a lock, and we own it, so we can clear it
+ * without atomic ops.
+ */
+ flags = atomic_read(&work->node.a_flags);
+ flags &= ~IRQ_WORK_PENDING;
+ atomic_set(&work->node.a_flags, flags);
+
+ /*
+ * See irq_work_claim().
+ */
+ smp_mb();
+
+ lockdep_irq_work_enter(flags);
+ work->func(work);
+ lockdep_irq_work_exit(flags);
+
+ /*
+ * Clear the BUSY bit, if set, and return to the free state if no-one
+ * else claimed it meanwhile.
+ */
+ (void)atomic_cmpxchg(&work->node.a_flags, flags, flags & ~IRQ_WORK_BUSY);
+}
+
+/*
+ * Synchronize against the irq_work @entry, ensures the entry is not
+ * currently in use.
+ */
+void irq_work_sync(struct irq_work *work)
+{
+ lockdep_assert_irqs_enabled();
+
+ while (irq_work_is_busy(work))
+ cpu_relax();
+}
+EXPORT_SYMBOL_GPL(irq_work_sync);
+
+#endif /* CONFIG_IRQ_WORK || CONFIG_SMP */
+
+#ifdef CONFIG_SMP
+
+static void __irq_work_queue_remote(int cpu, struct irq_work *work)
+{
+ /* Arch remote IPI send/receive backend aren't NMI safe */
+ WARN_ON_ONCE(in_nmi());
+ __smp_call_single_queue(cpu, &work->node.llist);
+}
+
+int irq_work_queue_remote(int cpu, struct irq_work *work)
+{
+ /*
+ * Ensures preemption is disabled in the caller.
+ */
+ WARN_ON_ONCE(cpu == smp_processor_id());
+
+ if (!irq_work_claim(work))
+ return -EBUSY;
+
+ __irq_work_queue_remote(cpu, work);
+
+ return 0;
+}
+
+#endif /* CONFIG_SMP */
+
+#ifdef CONFIG_IRQ_WORK
+
+static DEFINE_PER_CPU(struct llist_head, raised_list);
+static DEFINE_PER_CPU(struct llist_head, lazy_list);
+
void __weak arch_irq_work_raise(void)
{
/*
@@ -101,9 +174,7 @@ bool irq_work_queue_on(struct irq_work *

preempt_disable();
if (cpu != smp_processor_id()) {
- /* Arch remote IPI send/receive backend aren't NMI safe */
- WARN_ON_ONCE(in_nmi());
- __smp_call_single_queue(cpu, &work->node.llist);
+ __irq_work_queue_remote(cpu, work);
} else {
__irq_work_queue_local(work);
}
@@ -131,36 +202,6 @@ bool irq_work_needs_cpu(void)
return true;
}

-void irq_work_single(void *arg)
-{
- struct irq_work *work = arg;
- int flags;
-
- /*
- * Clear the PENDING bit, after this point the @work can be re-used.
- * The PENDING bit acts as a lock, and we own it, so we can clear it
- * without atomic ops.
- */
- flags = atomic_read(&work->node.a_flags);
- flags &= ~IRQ_WORK_PENDING;
- atomic_set(&work->node.a_flags, flags);
-
- /*
- * See irq_work_claim().
- */
- smp_mb();
-
- lockdep_irq_work_enter(flags);
- work->func(work);
- lockdep_irq_work_exit(flags);
-
- /*
- * Clear the BUSY bit, if set, and return to the free state if no-one
- * else claimed it meanwhile.
- */
- (void)atomic_cmpxchg(&work->node.a_flags, flags, flags & ~IRQ_WORK_BUSY);
-}
-
static void irq_work_run_list(struct llist_head *list)
{
struct irq_work *work, *tmp;
@@ -196,17 +237,5 @@ void irq_work_tick(void)
irq_work_run_list(this_cpu_ptr(&lazy_list));
}

-/*
- * Synchronize against the irq_work @entry, ensures the entry is not
- * currently in use.
- */
-void irq_work_sync(struct irq_work *work)
-{
- lockdep_assert_irqs_enabled();
-
- while (irq_work_is_busy(work))
- cpu_relax();
-}
-EXPORT_SYMBOL_GPL(irq_work_sync);
-
#endif /* CONFIG_IRQ_WORK */
+
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1284,13 +1284,15 @@ static int rcu_implicit_dynticks_qs(stru
resched_cpu(rdp->cpu);
WRITE_ONCE(rdp->last_fqs_resched, jiffies);
}
- if (IS_ENABLED(CONFIG_IRQ_WORK) &&
- !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
+#ifdef CONFIG_IRQ_WORK
+ // XXX should we use irq_work_queue_remote() ?
+ if (!rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
(rnp->ffmask & rdp->grpmask)) {
rdp->rcu_iw_pending = true;
rdp->rcu_iw_gp_seq = rnp->gp_seq;
irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
}
+#endif
}

return 0;