Re: [PATCH] cpuidle: avoid using smp_processor_id() in preemptible code (nr_iowait_cpu) v4

Andrew Morton
2010-06-30T19:58:53Z

On Fri, 25 Jun 2010 16:39:33 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, 2010-06-17 at 09:29 +0300, Sergey Senozhatsky wrote:
> > Fix
> >
> > BUG: using smp_processor_id() in preemptible [00000000] code: s2disk/3392
>
> > The initial fix was to use get_cpu/put_cpu in nr_iowait_cpu. However,
> > Arjan stated that "the bug is that it needs to be nr_iowait_cpu(int cpu)".
> >
> > This patch introduces nr_iowait_cpu(int cpu) and changes to its callers.
> >
> > Arjan also pointed out that we can't use get_cpu/put_cpu in update_ts_time_stats
> > since we "pick the current cpu, rather than the one denoted by ts" in that case.
> > To match given *ts and cpu denoted by *ts we use new field in the struct tick_sched: int cpu.
>
>
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index b232ccc..db14691 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -51,6 +51,7 @@ struct tick_sched {
> > unsigned long check_clocks;
> > enum tick_nohz_mode nohz_mode;
> > ktime_t idle_tick;
> > + int cpu;
> > int inidle;
> > int tick_stopped;
> > unsigned long idle_jiffies;
>
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 1d7b9bc..1907037 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -38,6 +38,9 @@ static ktime_t last_jiffies_update;
> >
> > struct tick_sched *tick_get_tick_sched(int cpu)
> > {
> > + /*FIXME: Arjan van de Ven:
> > + can we do this bit once, when the ts structure gets initialized?*/
> > + per_cpu(tick_cpu_sched, cpu).cpu = cpu;
> > return &per_cpu(tick_cpu_sched, cpu);
> > }
>
> > @@ -161,7 +164,7 @@ update_ts_time_stats(struct tick_sched *ts, ktime_t now, u64 *last_update_time)
> > if (ts->idle_active) {
> > delta = ktime_sub(now, ts->idle_entrytime);
> > ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> > - if (nr_iowait_cpu() > 0)
> > + if (nr_iowait_cpu(ts->cpu) > 0)
> > ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
> > ts->idle_entrytime = now;
> > }
>
>
> This all seems extremely silly, why not something like:

Does it work?

c'mon guys, it's taking us weeks and weeks to fix one simple bug. It's
a regression! We should be in panic mode.