Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

Peter Zijlstra
2010-12-10T19:26:18Z

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