[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>

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index fd2ff0698a851e213a4e0b1e7b814a3d338847e6..bbd386f572d97a6ed6cc8413f50d5e09ac019382 100644 (file)
--- 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 a8b387197d5bf79f9ac506f29cc42a9db1e40ecd..44238ab2fc99ff803836e263f42e7a2860fd3996 100644 (file)
--- 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 46bf5de5ac720f1691b75bce5ea022eb7cf7d874..af3b07dfad4be2fea39a75522347ac81ed0a8b3f 100644 (file)
--- 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)