[MIPS] SMTC: Instant IPI replay.

SMTC pseudo-interrupts between TCs are deferred and queued if the target
TC is interrupt-inhibited (IXMT). In the first SMTC prototypes, these
queued IPIs were serviced on return to user mode, or on entry into the
kernel idle loop. The INSTANT_REPLAY option dispatches them as part of
local_irq_restore() processing, which adds runtime overhead (hence the
option to turn it off), but ensures that IPIs are handled promptly even
under heavy I/O interrupt load.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -1568,6 +1568,20 @@ config MIPS_MT_FPAFF
 	depends on MIPS_MT
 	default y
 
+config MIPS_MT_SMTC_INSTANT_REPLAY
+	bool "Low-latency Dispatch of Deferred SMTC IPIs"
+	depends on MIPS_MT_SMTC
+	default y
+	help
+	  SMTC pseudo-interrupts between TCs are deferred and queued
+	  if the target TC is interrupt-inhibited (IXMT). In the first
+	  SMTC prototypes, these queued IPIs were serviced on return
+	  to user mode, or on entry into the kernel idle loop. The
+	  INSTANT_REPLAY option dispatches them as part of local_irq_restore()
+	  processing, which adds runtime overhead (hence the option to turn
+	  it off), but ensures that IPIs are handled promptly even under
+	  heavy I/O interrupt load.
+
 config MIPS_VPE_LOADER_TOM
 	bool "Load VPE program into memory hidden from linux"
 	depends on MIPS_VPE_LOADER

arch/mips/kernel/smtc.c

@@ -1017,6 +1017,33 @@ void setup_cross_vpe_interrupts(void)
  * SMTC-specific hacks invoked from elsewhere in the kernel.
  */
 
+void smtc_ipi_replay(void)
+{
+	/*
+	 * To the extent that we've ever turned interrupts off,
+	 * we may have accumulated deferred IPIs.  This is subtle.
+	 * If we use the smtc_ipi_qdepth() macro, we'll get an
+	 * exact number - but we'll also disable interrupts
+	 * and create a window of failure where a new IPI gets
+	 * queued after we test the depth but before we re-enable
+	 * interrupts. So long as IXMT never gets set, however,
+	 * we should be OK:  If we pick up something and dispatch
+	 * it here, that's great. If we see nothing, but concurrent
+	 * with this operation, another TC sends us an IPI, IXMT
+	 * is clear, and we'll handle it as a real pseudo-interrupt
+	 * and not a pseudo-pseudo interrupt.
+	 */
+	if (IPIQ[smp_processor_id()].depth > 0) {
+		struct smtc_ipi *pipi;
+		extern void self_ipi(struct smtc_ipi *);
+
+		while ((pipi = smtc_ipi_dq(&IPIQ[smp_processor_id()]))) {
+			self_ipi(pipi);
+			smtc_cpu_stats[smp_processor_id()].selfipis++;
+		}
+	}
+}
+
 void smtc_idle_loop_hook(void)
 {
 #ifdef SMTC_IDLE_HOOK_DEBUG
@@ -1113,29 +1140,14 @@ void smtc_idle_loop_hook(void)
 	if (pdb_msg != &id_ho_db_msg[0])
 		printk("CPU%d: %s", smp_processor_id(), id_ho_db_msg);
 #endif /* SMTC_IDLE_HOOK_DEBUG */
+
 	/*
-	 * To the extent that we've ever turned interrupts off,
-	 * we may have accumulated deferred IPIs.  This is subtle.
-	 * If we use the smtc_ipi_qdepth() macro, we'll get an
-	 * exact number - but we'll also disable interrupts
-	 * and create a window of failure where a new IPI gets
-	 * queued after we test the depth but before we re-enable
-	 * interrupts. So long as IXMT never gets set, however,
-	 * we should be OK:  If we pick up something and dispatch
-	 * it here, that's great. If we see nothing, but concurrent
-	 * with this operation, another TC sends us an IPI, IXMT
-	 * is clear, and we'll handle it as a real pseudo-interrupt
-	 * and not a pseudo-pseudo interrupt.
+	 * Replay any accumulated deferred IPIs. If "Instant Replay"
+	 * is in use, there should never be any.
 	 */
-	if (IPIQ[smp_processor_id()].depth > 0) {
-		struct smtc_ipi *pipi;
-		extern void self_ipi(struct smtc_ipi *);
-
-		if ((pipi = smtc_ipi_dq(&IPIQ[smp_processor_id()])) != NULL) {
-			self_ipi(pipi);
-			smtc_cpu_stats[smp_processor_id()].selfipis++;
-		}
-	}
+#ifndef CONFIG_MIPS_MT_SMTC_INSTANT_REPLAY
+	smtc_ipi_replay();
+#endif /* CONFIG_MIPS_MT_SMTC_INSTANT_REPLAY */
 }
 
 void smtc_soft_dump(void)

include/asm-mips/irqflags.h

@@ -15,6 +15,27 @@
 
 #include <asm/hazards.h>
 
+/*
+ * CONFIG_MIPS_MT_SMTC_INSTANT_REPLAY does prompt replay of deferred IPIs,
+ * at the cost of branch and call overhead on each local_irq_restore()
+ */
+
+#ifdef CONFIG_MIPS_MT_SMTC_INSTANT_REPLAY
+
+extern void smtc_ipi_replay(void);
+
+#define irq_restore_epilog(flags)				\
+do {								\
+	if (!(flags & 0x0400))					\
+		smtc_ipi_replay();				\
+} while (0)
+
+#else
+
+#define irq_restore_epilog(ignore) do { } while (0)
+
+#endif /* CONFIG_MIPS_MT_SMTC_INSTANT_REPLAY */
+
 __asm__ (
 	"	.macro	raw_local_irq_enable				\n"
 	"	.set	push						\n"
@@ -193,6 +214,7 @@ do {									\
 		: "=r" (__tmp1)						\
 		: "0" (flags)						\
 		: "memory");						\
+	irq_restore_epilog(flags);					\
 } while(0)
 
 static inline int raw_irqs_disabled_flags(unsigned long flags)