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SubjectRe: [PATCH] FRV: Use the generic IRQ stuff
Date

Okay... I've got some timings for FRV using my non-genirq code and using the
generic IRQ code.

Adding the attached patch (or similar) into the kernel, I see the following
timings:

do_IRQ->timer_int timer_int->do_IRQ
Cache Cycles Time Cycles Time
=============== ======= ======================= =====================
non-genirq Cold 380 1.1uS 216 600nS
Warm 114 320nS 96 270nS
genirq Cold 503 1.4uS 409 1.1uS
Warm 87 240nS 99 270nS

The TIMERL counter increments at core clock speed (360MHz), so each cycle is
approx 2.8nS. Each cycle executes on instruction or one pair of packed
instructions under ideal circumstances. Misjudged branches cost 4 clock
cycles.

So the genirq patch bites you on a cold cache, probably due to all the little
bits of unpredictable icache it's got to collect (chip ops and suchlike
dangling off function pointers), but wins a bit on the warm cache front.

The genirq timings above are with __do_IRQ() removed from the kernel.

I used gdb to collect the timings. The times are initially very high, but
quickly settle to constant lower values.

David
---
diff --git a/arch/frv/kernel/irq.c b/arch/frv/kernel/irq.c
index 5ac041c..8bcb980 100644
--- a/arch/frv/kernel/irq.c
+++ b/arch/frv/kernel/irq.c
@@ -134,6 +134,10 @@ static struct irq_chip frv_cpu_pic = {
.end = frv_cpupic_end,
};

+unsigned long irq_entered_at;
+unsigned long irq_handler_entered_at;
+unsigned long irq_exited_at;
+
/*
* handles all normal device IRQ's
* - registers are referred to by the __frame variable (GR28)
@@ -142,9 +146,15 @@ static struct irq_chip frv_cpu_pic = {
*/
asmlinkage void do_IRQ(void)
{
+ asm volatile ("bar" ::: "memory");
+ asm volatile ("movsg timerl,%0" : "=r"(irq_entered_at) :: "memory");
irq_enter();
generic_handle_irq(__get_IRL(), __frame);
irq_exit();
+ asm volatile ("bar" ::: "memory");
+ asm volatile ("movsg timerl,%0" : "=r"(irq_exited_at) :: "memory");
+ asm volatile ("bar" ::: "memory");
+ asm volatile ("break" ::: "memory");
}

/*
diff --git a/arch/frv/kernel/time.c b/arch/frv/kernel/time.c
index 3d0284b..772b409 100644
--- a/arch/frv/kernel/time.c
+++ b/arch/frv/kernel/time.c
@@ -52,12 +52,18 @@ static inline int set_rtc_mmss(unsigned
return -1;
}

+extern unsigned long irq_handler_entered_at;
+
/*
* timer_interrupt() needs to keep up the real-time clock,
* as well as call the "do_timer()" routine every clocktick
*/
static irqreturn_t timer_interrupt(int irq, void *dummy, struct pt_regs * regs)
{
+ asm volatile ("movsg timerl,%0" : "=r"(irq_handler_entered_at) :: "memory");
+ return IRQ_HANDLED;
+
+#if 0
/* last time the cmos clock got updated */
static long last_rtc_update = 0;

@@ -98,6 +104,7 @@ #endif /* CONFIG_HEARTBEAT */

write_sequnlock(&xtime_lock);
return IRQ_HANDLED;
+#endif
}

void time_divisor_init(void)
-
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