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SubjectRe: Severe IRQ problems on Foster (P4 Xeon) system

On Wed, 13 Mar 2002, Martin Wilck wrote:

> First of all, we see that virtually 100% of all IRQs are handled by
> CPU 0. I have seen this reported a number of times before. I guess it
> can become a severe performance problem in IRQ-intensive situations.

i've written a patch for this, it's enclosed in this email. It implements
a brownean motion of IRQs, based on load patterns. The concept works
really well on Foster CPUs - eg. it will redirect IRQs to idle CPUs - but
if all CPUs are idle then the IRQs are randomly and evenly distributed
between CPUs.

(the patch can be made cheaper, but i've kept the overhead per-IRQ for the
time being to have more flexibility.)

let me know whether this fixes your problem,

Ingo

--- linux/kernel/sched.c.orig Tue Feb 5 13:11:35 2002
+++ linux/kernel/sched.c Tue Feb 5 13:12:48 2002
@@ -118,6 +118,11 @@
#define can_schedule(p,cpu) \
((p)->cpus_runnable & (p)->cpus_allowed & (1 << cpu))

+int idle_cpu(int cpu)
+{
+ return cpu_curr(cpu) == idle_task(cpu);
+}
+
#else

#define idle_task(cpu) (&init_task)
--- linux/include/linux/sched.h.orig Tue Feb 5 13:13:09 2002
+++ linux/include/linux/sched.h Tue Feb 5 13:14:00 2002
@@ -144,6 +144,7 @@

extern void sched_init(void);
extern void init_idle(void);
+extern int idle_cpu(int cpu);
extern void show_state(void);
extern void cpu_init (void);
extern void trap_init(void);
--- linux/include/asm-i386/hardirq.h.orig Tue Feb 5 13:10:39 2002
+++ linux/include/asm-i386/hardirq.h Tue Feb 5 13:14:00 2002
@@ -12,6 +12,7 @@
unsigned int __local_bh_count;
unsigned int __syscall_count;
struct task_struct * __ksoftirqd_task; /* waitqueue is too large */
+ unsigned long idle_timestamp;
unsigned int __nmi_count; /* arch dependent */
} ____cacheline_aligned irq_cpustat_t;

--- linux/arch/i386/kernel/io_apic.c.orig Tue Feb 5 13:10:37 2002
+++ linux/arch/i386/kernel/io_apic.c Tue Feb 5 13:15:23 2002
@@ -28,6 +28,7 @@
#include <linux/config.h>
#include <linux/smp_lock.h>
#include <linux/mc146818rtc.h>
+#include <linux/compiler.h>

#include <asm/io.h>
#include <asm/smp.h>
@@ -163,6 +164,86 @@
clear_IO_APIC_pin(apic, pin);
}

+static void set_ioapic_affinity (unsigned int irq, unsigned long mask)
+{
+ unsigned long flags;
+
+ /*
+ * Only the first 8 bits are valid.
+ */
+ mask = mask << 24;
+ spin_lock_irqsave(&ioapic_lock, flags);
+ __DO_ACTION(1, = mask, )
+ spin_unlock_irqrestore(&ioapic_lock, flags);
+}
+
+#if CONFIG_SMP
+
+typedef struct {
+ unsigned int cpu;
+ unsigned long timestamp;
+} ____cacheline_aligned irq_balance_t;
+
+static irq_balance_t irq_balance[NR_IRQS] __cacheline_aligned
+ = { [ 0 ... NR_IRQS-1 ] = { 1, 0 } };
+
+extern unsigned long irq_affinity [NR_IRQS];
+
+#endif
+
+#define IDLE_ENOUGH(cpu,now) \
+ (idle_cpu(cpu) && ((now) - irq_stat[(cpu)].idle_timestamp > 1))
+
+#define IRQ_ALLOWED(cpu,allowed_mask) \
+ ((1 << cpu) & (allowed_mask))
+
+static unsigned long move(int curr_cpu, unsigned long allowed_mask, unsigned long now, int direction)
+{
+ int search_idle = 1;
+ int cpu = curr_cpu;
+
+ goto inside;
+
+ do {
+ if (unlikely(cpu == curr_cpu))
+ search_idle = 0;
+inside:
+ if (direction == 1) {
+ cpu++;
+ if (cpu >= smp_num_cpus)
+ cpu = 0;
+ } else {
+ cpu--;
+ if (cpu == -1)
+ cpu = smp_num_cpus-1;
+ }
+ } while (!IRQ_ALLOWED(cpu,allowed_mask) ||
+ (search_idle && !IDLE_ENOUGH(cpu,now)));
+
+ return cpu;
+}
+
+static inline void balance_irq(int irq)
+{
+#if CONFIG_SMP
+ irq_balance_t *entry = irq_balance + irq;
+ unsigned long now = jiffies;
+
+ if (unlikely(entry->timestamp != now)) {
+ unsigned long allowed_mask;
+ int random_number;
+
+ rdtscl(random_number);
+ random_number &= 1;
+
+ allowed_mask = cpu_online_map & irq_affinity[irq];
+ entry->timestamp = now;
+ entry->cpu = move(entry->cpu, allowed_mask, now, random_number);
+ set_ioapic_affinity(irq, 1 << entry->cpu);
+ }
+#endif
+}
+
/*
* support for broken MP BIOSs, enables hand-redirection of PIRQ0-7 to
* specific CPU-side IRQs.
@@ -653,8 +734,7 @@
}

/*
- * Set up the 8259A-master output pin as broadcast to all
- * CPUs.
+ * Set up the 8259A-master output pin:
*/
void __init setup_ExtINT_IRQ0_pin(unsigned int pin, int vector)
{
@@ -1174,6 +1254,7 @@
*/
static void ack_edge_ioapic_irq(unsigned int irq)
{
+ balance_irq(irq);
if ((irq_desc[irq].status & (IRQ_PENDING | IRQ_DISABLED))
== (IRQ_PENDING | IRQ_DISABLED))
mask_IO_APIC_irq(irq);
@@ -1213,6 +1294,7 @@
unsigned long v;
int i;

+ balance_irq(irq);
/*
* It appears there is an erratum which affects at least version 0x11
* of I/O APIC (that's the 82093AA and cores integrated into various
@@ -1268,19 +1350,6 @@
}

static void mask_and_ack_level_ioapic_irq (unsigned int irq) { /* nothing */ }
-
-static void set_ioapic_affinity (unsigned int irq, unsigned long mask)
-{
- unsigned long flags;
- /*
- * Only the first 8 bits are valid.
- */
- mask = mask << 24;
-
- spin_lock_irqsave(&ioapic_lock, flags);
- __DO_ACTION(1, = mask, )
- spin_unlock_irqrestore(&ioapic_lock, flags);
-}

/*
* Level and edge triggered IO-APIC interrupts need different handling,
--- linux/arch/i386/kernel/irq.c.orig Tue Feb 5 13:10:34 2002
+++ linux/arch/i386/kernel/irq.c Tue Feb 5 13:11:15 2002
@@ -1076,7 +1076,7 @@

static struct proc_dir_entry * smp_affinity_entry [NR_IRQS];

-static unsigned long irq_affinity [NR_IRQS] = { [0 ... NR_IRQS-1] = ~0UL };
+unsigned long irq_affinity [NR_IRQS] = { [0 ... NR_IRQS-1] = ~0UL };
static int irq_affinity_read_proc (char *page, char **start, off_t off,
int count, int *eof, void *data)
{
-
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