irq_work, smp: Allow irq_work on call_single_queue

Currently irq_work_queue_on() will issue an unconditional
arch_send_call_function_single_ipi() and has the handler do
irq_work_run().

This is unfortunate in that it makes the IPI handler look at a second
cacheline and it misses the opportunity to avoid the IPI. Instead note
that struct irq_work and struct __call_single_data are very similar in
layout, so use a few bits in the flags word to encode a type and stick
the irq_work on the call_single_queue list.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200526161908.011635912@infradead.org

include/linux/irq_work.h

@@ -13,6 +13,8 @@
  * busy      NULL, 2 -> {free, claimed} : callback in progress, can be claimed
  */
 
+/* flags share CSD_FLAG_ space */
+
 #define IRQ_WORK_PENDING	BIT(0)
 #define IRQ_WORK_BUSY		BIT(1)
 
@@ -23,9 +25,12 @@
 
 #define IRQ_WORK_CLAIMED	(IRQ_WORK_PENDING | IRQ_WORK_BUSY)
 
+/*
+ * structure shares layout with single_call_data_t.
+ */
 struct irq_work {
-	atomic_t flags;
 	struct llist_node llnode;
+	atomic_t flags;
 	void (*func)(struct irq_work *);
 };