nohz: Introduce arch_needs_cpu

Allow the architecture to request a normal jiffy tick when the system
goes idle and tick_nohz_stop_sched_tick is called . On s390 the hook is
used to prevent the system going fully idle if there has been an
interrupt other than a clock comparator interrupt since the last wakeup.

On s390 the HiperSockets response time for 1 connection ping-pong goes
down from 42 to 34 microseconds. The CPU cost decreases by 27%.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
LKML-Reference: <20090929122533.402715150@de.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

3 files changed, 12 insertions, 0 deletions

diff --git a/arch/s390/include/asm/cputime.h b/arch/s390/include/asm/cputime.h
index 24b1244aadb9..95f3561517c7 100644
--- a/arch/s390/include/asm/cputime.h
+++ b/arch/s390/include/asm/cputime.h
@@ -183,6 +183,7 @@ struct s390_idle_data {
         unsigned long long idle_count;
         unsigned long long idle_enter;
         unsigned long long idle_time;
+        int nohz_delay;
 };
 
 DECLARE_PER_CPU(struct s390_idle_data, s390_idle);
@@ -198,4 +199,11 @@ static inline void s390_idle_check(void)
                 vtime_start_cpu();
 }
 
+static inline int s390_nohz_delay(int cpu)
+{
+        return per_cpu(s390_idle, cpu).nohz_delay != 0;
+}
+
+#define arch_needs_cpu(cpu) s390_nohz_delay(cpu)
+
 #endif /* _S390_CPUTIME_H */
diff --git a/arch/s390/kernel/s390_ext.c b/arch/s390/kernel/s390_ext.c
index 0de305b598ce..59618bcd99b7 100644
--- a/arch/s390/kernel/s390_ext.c
+++ b/arch/s390/kernel/s390_ext.c
@@ -126,6 +126,8 @@ void __irq_entry do_extint(struct pt_regs *regs, unsigned short code)
                 /* Serve timer interrupts first. */
                 clock_comparator_work();
         kstat_cpu(smp_processor_id()).irqs[EXTERNAL_INTERRUPT]++;
+        if (code != 0x1004)
+                __get_cpu_var(s390_idle).nohz_delay = 1;
         index = ext_hash(code);
         for (p = ext_int_hash[index]; p; p = p->next) {
                 if (likely(p->code == code))
diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c
index c41bb0d416e1..b59a812a010e 100644
--- a/arch/s390/kernel/vtime.c
+++ b/arch/s390/kernel/vtime.c
@@ -167,6 +167,8 @@ void vtime_stop_cpu(void)
         /* Wait for external, I/O or machine check interrupt. */
         psw.mask = psw_kernel_bits | PSW_MASK_WAIT | PSW_MASK_IO | PSW_MASK_EXT;
 
+        idle->nohz_delay = 0;
+
         /* Check if the CPU timer needs to be reprogrammed. */
         if (vq->do_spt) {
                 __u64 vmax = VTIMER_MAX_SLICE;