[BUG] 2.6.37-rc3 massive interactivity regression on ARM

[peterz@infradead.org (Peter Zijlstra)]

On Fri, 2010-12-10 at 19:10 +0100, Peter Zijlstra wrote:
> > > +   delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
> > > +   rq->clock += delta;
> > 
> > Hmm.  Can you tell me how this is different to:
> > 
> >       new_clock = sched_clock_cpu(cpu_of(rq));
> >       delta = new_clock - rq->clock;
> >       rq->clock = new_clock;
> > 
> > which I think may be simpler in terms of 64-bit math for 32-bit compilers
> > to deal with?
> 
> Its not, I could write it like that, the only reason I didn't is because
> it uses an extra variable. If gcc on 32bit targets really generates
> hideous code for it I'll happily change it. 

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -1863,6 +1863,7 @@ void account_system_vtime(struct task_st
 {
        unsigned long flags;
        s64 delta;
+       u64 now;
        int cpu;
 
        if (!sched_clock_irqtime)
@@ -1870,8 +1871,9 @@ void account_system_vtime(struct task_st
 
        local_irq_save(flags);
        cpu = smp_processor_id();
-       delta = sched_clock_cpu(cpu) - per_cpu(irq_start_time, cpu);
-       per_cpu(irq_start_time, cpu) += delta;
+       now = sched_clock_cpu(cpu);
+       delta = now - per_cpu(irq_start_time, cpu);
+       per_cpu(irq_start_time, cpu) = now;
 
        irq_time_write_begin(cpu);
 
On i386 (gcc (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5):

Before: account_system_vtime: 160 bytes
After:  account_system_vtime: 214 bytes