[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>

3 files changed, 69 insertions, 21 deletions

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index fd2ff0698a85..bbd386f572d9 100644
--- a/arch/mips/Kconfig
+++ b/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
diff --git a/arch/mips/kernel/smtc.c b/arch/mips/kernel/smtc.c
index a8b387197d5b..44238ab2fc99 100644
--- a/arch/mips/kernel/smtc.c
+++ b/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)
diff --git a/include/asm-mips/irqflags.h b/include/asm-mips/irqflags.h
index 46bf5de5ac72..af3b07dfad4b 100644
--- a/include/asm-mips/irqflags.h
+++ b/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)